CI: tweak both manifest and manifest-rules workflow rules

.gitlab-ci.yml

@@ -3,10 +3,12 @@ stages:
   - check-version
 
 
-# Don't run on merge requests
+# Don't run pipeline on MRs
 workflow:
   rules:
-    - if: '$CI_MERGE_REQUEST_ID == null'
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+      when: never
+    - when: always
 
 
 test: