ci: use detached MR pipelines

See the freedesktop wiki [1]. This allows external contributors to
have CI run properly.

[1]: https://gitlab.freedesktop.org/freedesktop/freedesktop/-/wikis/GitLab-CI#for-project-developers

Signed-off-by: Simon Ser <contact@emersion.fr>

.gitlab-ci.yml

@@ -62,6 +62,13 @@ stages:
   - "Build and test"
   - "Other build configurations"
 
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
+    - if: $CI_COMMIT_BRANCH
+
 .ci-rules:
   rules:
     - when: on_success