Fix current host IP address being reported as remote host

[MetRonnie]

Supersedes MetRonnie#36

Fixes bug uncovered in #6745:

Test failure tests/functional/workflow-host-self-id/00-address.t seems to be an uncovered bug:

>>> from cylc.flow.hostuserutil import is_remote_host, get_host_ip_by_name
>>> is_remote_host(get_host_ip_by_name('localhost'))  # (127.0.0.1 loopback IP address)
True
>>> is_remote_host(get_host_ip_by_name(get_host()))  # (network-accessible IP address)
True

In other words, when [scheduling][host self-identification]method = address, is_remote_host() gives false positives.

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• No dependency changes
• Tests are included
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• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.