Flaky test: TestRunWithSystemdTrueEnabled

[AkihiroSuda]

=== Failed
=== FAIL: cmd/nerdctl/container TestRunWithSystemdTrueEnabled/waits_for_systemd_to_become_ready_and_lists_systemd_jobs (0.04s)
    container_run_systemd_linux_test.go:120: 
        
        +============================================================================================================+
        | 🚀      | "TestRunWithSystemdTrueEnabled/waits_for_systemd_to_become_ready_and_lists_systemd_jobs": starti |
        |         | ng test!                                                                                         |
        +============================================================================================================+
        | ⏳      | /tmp/TestRunWithSystemdTrueEnabledwaits_for_systemd_to_become_ready_208478251/001/aab6ed55       |
        +============================================================================================================+
        |         |                                                                                                  |
        +============================================================================================================+
        | config  | {                                                                                                |
        |         |   "Namespace": "nerdctl-test",                                                                   |
        |         |   "PrivateMode": ""                                                                              |
        |         | }                                                                                                |
        +============================================================================================================+
        | labels  | {}                                                                                               |
        +============================================================================================================+
        
    container_run_systemd_linux_test.go:120: 
        
        +------------------------------------------------------------------------------------------------------------+
        | ➡️      | ⚙️ /usr/local/bin/nerdctl exec nerdctl-testrunwithsystemdtrueenabled sh -c -- tries=0            |
        |         |                                                                                                  |
        |         |       until systemctl is-system-running >/dev/null 2>&1; do                                      |
        |         |                                                                                                  |
        |         |         >&2 printf "Waiting for systemd to come up...\n"                                         |
        |         |         sleep 1s                                                                                 |
        |         |         tries=$(( tries + 1))                                                                    |
        |         |         [ $tries -lt 10 ] || {                                                                   |
        |         |           >&2 printf "systemd failed to come up in a reasonable amount of time\n"                |
        |         |           exit 1                                                                                 |
        |         |         }                                                                                        |
        |         |       done                                                                                       |
        |         |       systemctl list-jobs                                                                        |
        +------------------------------------------------------------------------------------------------------------+
        |         | 🚫 command returned a non-zero exit code                                                         |
        +------------------------------------------------------------------------------------------------------------+
        |         | ⚠️ 1                                                                                             |
        +------------------------------------------------------------------------------------------------------------+
        |         | 🟠 time="2026-02-15T06:17:31Z" level=fatal msg="OCI runtime exec failed: exec failed: unable to  |
        |         | start container process: error starting setns process: exec: already started"                    |
        +------------------------------------------------------------------------------------------------------------+
        | 🌱      | HOME=/root                                                                                       |
        |         | PATH=/usr/local/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin |
        |         | :/bin                                                                                            |
        |         | DOCKER_CONFIG=/tmp/TestRunWithSystemdTrueEnabledwaits_for_systemd_to_become_ready_208478251/003  |
        |         | NERDCTL_TOML=/tmp/TestRunWithSystemdTrueEnabledwaits_for_systemd_to_become_ready_208478251/003/n |
        |         | erdctl.toml                                                                                      |
        +------------------------------------------------------------------------------------------------------------+
        | ⏰      | <1s (limit: 3m0s)                                                                                |
        +------------------------------------------------------------------------------------------------------------+
        | 📁      | /tmp/TestRunWithSystemdTrueEnabledwaits_for_systemd_to_become_ready_208478251/002                |
        +------------------------------------------------------------------------------------------------------------+
        
    container_run_systemd_linux_test.go:120: 	
        
        <<<<<<<<<<<<<<<<<<<<
        	🖊️ Command should succeed
        	👀 testing:		`command returned a non-zero exit code`
        	❌ FAILED!		is `<nil>`
        >>>>>>>>>>>>>>>>>>>>

https://github.com/containerd/nerdctl/actions/runs/21416963179/job/63655402439?pr=4704

Probably unrelated to this PR. Let's merge this

Originally posted by @AkihiroSuda in #4704 (comment)