Manifest: switch from branch pipelines to merge request pipelines

This will make the manifest project support the "skip build" label.

manifest-pipeline.yml

@@ -16,9 +16,6 @@ workflow:
     - if: $CI_PIPELINE_SOURCE == "api"
     - if: $CI_PIPELINE_SOURCE == "pipeline"
     - if: $CI_PIPELINE_SOURCE == "web"
-    # Do not run pipelines for merge requests
-    - if: $CI_MERGE_REQUEST_IID
-      when: never
     # Do not run pipelines on forked projects
     # (use id instead of name because of rename)
     - if: $CI_PROJECT_ID != $MANIFEST_PROJECT_ID
@@ -26,14 +23,16 @@ workflow:
     # Do not run pipelines on integration branches
     - if: $CI_COMMIT_REF_NAME =~ /^integrate\/.*/
       when: never
-    # In all other cases, run the pipeline automatically
-    - when: always
+    # Run pipelines for merge requests
+    - if: $CI_MERGE_REQUEST_IID
+    # Run pipelines on the master branch
+    - if: $CI_COMMIT_REF_NAME == $MASTER_BRANCH
 
 .full_build_pipeline:
   rules:
-    # Run the full build pipeline on non-master branches (i.e. in the merge request)
+    # Run the full build pipeline on merge requests
     # or if explicitly triggered by the API or the web button.
-    - if: $CI_COMMIT_REF_NAME != $MASTER_BRANCH
+    - if: $CI_MERGE_REQUEST_IID
     - if: $CI_PIPELINE_SOURCE == "api"
     - if: $CI_PIPELINE_SOURCE == "pipeline"
     - if: $CI_PIPELINE_SOURCE == "web"