ci: Don't forward default variables to downstream cerbero pipeline

Forward only the specified variables. Fixes an issue where the wrong
images were used for cerbero trigger pipeline jobs.

This list needs to be kept up-to-date whenever the rules: below it
change.

https://docs.gitlab.com/ci/yaml/#triggerforward

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9356>

.gitlab-ci.yml

@@ -1091,6 +1091,11 @@ cerbero trigger:
     CI_GSTREAMER_PATH: '$CI_PROJECT_PATH'
     CI_GSTREAMER_REF_NAME: '$CI_COMMIT_REF_NAME'
     CI_GSTREAMER_TRIGGERED: 'true'
+  inherit:
+    # Inherit default (not predefined) variables used for the job in `rules:` and `variables:`
+    variables:
+      - MESON_COMMIT
+      - GST_UPSTREAM_BRANCH
   rules:
     - if: '$MESON_COMMIT'
     # Never run post merge