ci: Add a job to trigger a cerbero build, similar to the monorepo

This should help prevent breakage like cerbero!1418 (merged).

.gitlab-ci.yml

@@ -387,9 +387,7 @@ coverage:
         coverage_format: cobertura
         path: coverage.xml
 
-# XXX: Copied from gstreamer monorepo, keep in sync
-cerbero trigger:
-  timeout: '4h'
+meson shared2:
   image: $CERBERO_TRIGGER_IMAGE
   needs: [ "trigger" ]
   variables:
@@ -418,3 +416,4 @@ cerbero trigger:
       - "ci/*.sh"
       - "ci/*.py"
       when: never
+    - when: always