gitlab-ci: Only trigger test-docs job automatically for MRs

Follow-up to
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5898 .

Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5918>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 45fac9f..594cadb 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -69,7 +69,7 @@
     - *ignore_scheduled_pipelines
     - if: '$CI_PROJECT_NAMESPACE == "mesa"'
       when: never
-    - if: '$GITLAB_USER_LOGIN == "marge-bot"'
+    - if: '$GITLAB_USER_LOGIN == "marge-bot" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME == $CI_COMMIT_REF_NAME'
       changes: *docs-or-ci
       when: on_success
     - changes: *docs-or-ci