github: fix Meson tests not executing at all

github: fix Meson tests not executing at all

While the win+Meson test jobs run in GitHub workflows, the shell script
that is supposed to run the jobs is seemingly not running at all. All
that the CI job prints is the following:

  Run ci/run-test-slice-meson.sh build 1 10
    ci/run-test-slice-meson.sh build 1 10
    shell: C:\Program Files\PowerShell\7\pwsh.EXE -command ". '{0}'"
    env:
      DEVELOPER: 1

The step is currently defined to use PowerShell, and of course it
doesn't know how to execute POSIX shell scripts. What's surprising
though is that this step doesn't even lead to a CI failure.

Fix the issue by using Bash instead of PowerShell, as we do in other
steps that execute shell scripts.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>