ci: Add "is forked branch" YAML anchor

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6566>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index d8f916a..cd9e9d5 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -44,6 +44,9 @@
 # --------------------------------
 .rules-anchors:
   rules:
+    # Forked project branch
+    - if: &is-forked-branch '$CI_PROJECT_PATH != "mesa/mesa" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME != $CI_COMMIT_REF_NAME'
+      when: manual
     # Forked project branch / pre-merge pipeline
     - if: &is-forked-branch-or-pre-merge '$CI_PROJECT_PATH != "mesa/mesa" || $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME == $CI_COMMIT_REF_NAME'
       when: manual
@@ -1077,7 +1080,7 @@
 .test-manual:
   rules:
     - *ignore_scheduled_pipelines
-    - if: '$CI_PROJECT_PATH != "mesa/mesa" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME != $CI_COMMIT_REF_NAME'
+    - if: *is-forked-branch
       changes:
         *all_paths
       when: manual