citemplate: only spawn a single pipeline in Merge Requests

This is the 3rd or 4th time we are trying this yes, (!331),
but hear me out. This time is for Real!

Jokes aside, its possible to avoid spawning >1 pipeline per event
with gitlab 13.8.

https://gitlab.com/gitlab-org/gitlab/-/issues/201845

https://gitlab.com/gitlab-org/gitlab/-/issues/299409

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-ci/-/merge_requests/391>

gitlab/ci_template.yml

@@ -91,8 +91,13 @@ variables:
 workflow:
   rules:
     - if: $CI_MERGE_REQUEST_IID
+    # don't create a pipeline if its a commit pipeline, on a branch and that branch has
+    # open merge requests (bc we will get a MR build instead)
+    - if: $CI_OPEN_MERGE_REQUESTS
+      when: never
     - if: $CI_COMMIT_TAG
     - if: $CI_COMMIT_BRANCH
+
 #
 # Global CI policy
 #