gitlab: purge msys pacman cache

gitlab: purge msys pacman cache

For the Windows msys2 CI job we install many packages using pacman
and use the GitLab cache to preserve the pacman cache across CI
runs. While metadata still needs downloading, this avoids pacman
re-downloading packages from msys2 if they have not changed.

The problem is that pacman never automatically purges anything
from its package cache. Thus the GitLab cache is growing without
bound and packing/unpacking the cache is consuming an increasing
amount of time in the CI job.

If we run 'pacman -Sc' /after/ installing our desired package set,
it will purge any cached downloaded packages that are not matching
any installed package.

This will (currently) cap the pacman download cache at approx
256 MB.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-ID: <20251010160545.144760-1-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>