ci: Do not create pipelines for branches with open MRs

Pipelines for non-protected branches are set to 'manual', and
thus cheap. However they may still get picked by `@marge-bot`,
meaning that the bot waits for the completion of a pipeline that
never starts.

Avoid that by not creating pipelines for branches with open
merge requests.

Credit to Jordan, who came up with this for gst.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/300>

.gitlab-ci.yml

@@ -32,6 +32,8 @@ variables:
 
 workflow:
   rules:
+    - if: '$CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS && $CI_PIPELINE_SOURCE == "push"'
+      when: never
     - if: '$CI_MERGE_REQUEST_IID'
     - if: '$CI_COMMIT_TAG'
     - if: '$CI_COMMIT_BRANCH'