ci: Restrict "success" job to pipelines for MRs

Expected benefits:

* No more spurious non-MR pipelines with just this job.
* No more warnings from GitLab such as on
  https://gitlab.freedesktop.org/mesa/mesa/-/pipelines/195778

Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6492>