ci: Don't exclude "success" job from mesa/mesa pipelines for MRs

Since the last GitLab update, pre-merge pipelines for MRs run in the
mesa/mesa namespace, so this job didn't get created anymore, causing
trouble.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6522>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 13a9cae..35593d7 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -119,7 +119,7 @@
   image: debian:stable-slim
   rules:
     - *ignore_scheduled_pipelines
-    - if: '$CI_PROJECT_NAMESPACE == "mesa"'
+    - if: '$CI_PROJECT_NAMESPACE == "mesa" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME != $CI_COMMIT_REF_NAME'
       when: never
     - if: '$GITLAB_USER_LOGIN == "marge-bot"'
       changes: *docs-or-ci