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>