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>

.gitlab-ci.yml

@@ -762,6 +762,8 @@ build documentation:
 cerbero trigger:
   stage: build
   timeout: '3h'
+  tags:
+    - placeholder-job
   extends:
     - '.fedora image'
     - '.fdo.suffixed-image@fedora'