[Bug 56185] New: The GitLab Windows test job runs even when not needed

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=56185

Bug ID: 56185
           Summary: The GitLab Windows test job runs even when not needed
           Product: WineHQ Gitlab
           Version: unspecified
          Hardware: x86-64
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gitlab-unknown
          Assignee: [e-mail]
          Reporter: [e-mail]

The GitLab Windows test jobs run even for MRs that don´t touch the tests.
For instance MR!4840 does not touch the test:
https://gitlab.winehq.org/wine/wine/-/merge_requests/4840/diffs

And yet test-win10-21h2-32 ran (same for the 64-bit tests job):
https://gitlab.winehq.org/fgouget/wine/-/jobs/48245

And the corresponding winetest.log file is empty which corresponds to an empty
winetest.args file. This makes sense since build-linux systematically generates
winetest.args and it's only in the Windows job that we check whether that file
is empty or not.

This could be avoided by generating dynamic child jobs in build-linux.
Then the check for whether it is necessary to run the Windows tests could be
done in build-linux and a 'dummy: rules: when: never' job could be generated if
nothing needs to be run.

See below for an example of a dynamic child job. 
https://blog.devops.dev/using-dynamic-pipelines-in-gitlab-583cf4f6acd0