gitlab-ci: Fix "triggered by Marge for a merge request" rule

The commit below changed the rule such that it accidentally also applied
to the non-MR pipelines created by Marge, resulting in Marge triggering
twice as many jobs as necessary.

Fixes: 549b4a3dd4ab "gitlab-ci: Automatically run pipelines for Marge
                     Bot pre-merge only"

Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5898>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 82a2abf..e769069 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -173,7 +173,7 @@
       when: on_success
     # Run pipeline by default if it was triggered by Marge Bot, is for a
     # merge request, and any files affecting the pipeline were changed
-    - if: '$GITLAB_USER_LOGIN == "marge-bot" && $CI_PROJECT_PATH != "mesa/mesa"'
+    - if: '$GITLAB_USER_LOGIN == "marge-bot" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME == $CI_COMMIT_REF_NAME'
       changes:
         *all_paths
       when: on_success