ci: Don't tie up a full job slot for cerbero trigger

ci: Don't tie up a full job slot for cerbero trigger

We don't have a huge amount of capacity on shared runners, so better if
we don't use that limited capacity on a job which sits there doing
~nothing.

There is a new runner class called 'placeholder-job' which accepts a
huge number of parallel jobs, separately to normal jobs, on the
assumption that they will consume ~no resources; start using that.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2071>