Add workflow rules

.gitlab-ci.yml

@@ -3,6 +3,16 @@ stages:
   - check
   - trigger
 
+workflow:
+  rules:
+    # Run when a merge request event is open
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    # Do not run branch-specific pipeline if there is an open MR
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
+    # Run reduced pipeline for branches
+    - if: $CI_COMMIT_BRANCH
+
 include:
   # Low-effort, we can always do lint checks, validations, and so forth
   - local: .gitlab/always.yml