Revert "Revert "ci: check commit using ci-fairy""

This reverts commit d4e55b65.

Previously, we had an issue where the ci-fairy would check commits
that were not part of the MR. This was most likely caused due to
the multi-repo setup we had and we shouldn't be hitting it anymore
with the monorepo.

See:

gstreamer/gst-ci!304

and

gstreamer/gst-ci!383

This time we also limit the job to detached pipelines