.gitlab-ci: Do builds under /builds as this is never an overlayfs

.gitlab-ci: Do builds under /builds as this is never an overlayfs

On the GitLab shared runners / is overlayfs, which /builds being ext,
so we want this real filesystem, which should be faster in any case.

This may allow us to use GitLab shared runners for more jobs.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Tue Sep  5 14:20:48 UTC 2023 on atb-devel-224