add workflow rules

.gitlab-ci.yml

 image: "rust:latest"
 
+workflow:
+  rules:
+    - if: $CI_COMMIT_MESSAGE =~ /^(draft|no-ci):/
+      when: never
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
+    - if: ($CI_PIPELINE_SOURCE == "push") && ($CI_COMMIT_BRANCH == "master")
+
 format:
   script:
     - cargo fmt --all --check