ci: Rename only_default to pipeline_guard

It's the rules that guard the pipelines, deciding when they are allowed
to run or not. The 'only_default' comes from the fact that in the past
they set the only: field.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1588>

.gitlab-ci.yml

@@ -58,7 +58,7 @@ workflow:
     - if: '$CI_COMMIT_TAG'
     - if: '$CI_COMMIT_BRANCH'
 
-.only_default: &only_default
+.pipline-guard: &pipline-guard
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
     - if: '$CI_COMMIT_TAG'
@@ -74,7 +74,7 @@ build-fedora-container:
   needs:
     - check-commit-log
     - check-merge-request
-  <<: *only_default
+  &pipline-guard
 
 check-commit-log:
   extends: