ci: Use !reference tag for pipeline guard

It's a bit more flexible than yaml anchors, and already used in
mutter for the same purpose.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3370>

.gitlab-ci.yml

@@ -43,7 +43,7 @@ workflow:
     - if: '$CI_COMMIT_TAG'
     - if: '$CI_COMMIT_BRANCH'
 
-.pipeline_guard: &pipeline_guard
+.pipeline-guard:
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
     - if: '$CI_COMMIT_TAG'
@@ -94,7 +94,8 @@ check_commit_log:
       else
         echo "Not a merge request" ;
       fi
-  <<: *pipeline_guard
+  rules:
+    - !reference [.pipeline-guard, rules]
   artifacts:
     expire_in: 1 week
     paths:
@@ -113,7 +114,8 @@ check-merge-request:
       else
         echo "Not a merge request" ;
       fi
-  <<: *pipeline_guard
+  rules:
+    - !reference [.pipeline-guard, rules]
   artifacts:
     expire_in: 1 week
     paths: