chore(ci): Split pre-release tests into cron job, open issues on failure

[effigies]

In #3745, the pre-release tests are failing, causing other the PR to fail. This is generally not desirable, as these are rarely the fault of contributor PRs. This pulls the prerelease tests into their own workflow, run on a schedule rather than PR. The ci/prerelease branch can be used to debug and fix issues.

@Remi-Gau helped me set this up in nibabel.

Incidentally, also split the install and test tox stages. Sometimes install can fail or take a long time due to infrastructure issues, and it's nice to separate that from test failures at a glance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.88%. Comparing base (b8676b5) to head (21b971b).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3746   +/-   ##
=======================================
  Coverage   72.88%   72.88%           
=======================================
  Files        1279     1279           
  Lines       59351    59351           
=======================================
  Hits        43256    43256           
  Misses      16095    16095           

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