ci: auto exec job fmt_and_clippy for MRs only

.gitlab-ci.yml

@@ -23,8 +23,8 @@ fmt_and_clippy:
   rules:
     - if: $CI_COMMIT_TAG
       when: never
-    - if: $CI_MERGE_REQUEST_ID
-    - when: on_success
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - when: manual
   stage: quality
   script:
     - cargo fmt -- --version