ci: Use detached pipelines

See https://gitlab.freedesktop.org/freedesktop/freedesktop/-/issues/438#what-it-means-for-me-a-maintainer-of-a-project-part-of-gitlabfreedesktoporg

.gitlab-ci.yml

@@ -18,6 +18,11 @@ variables:
                 python3-psutil
                 umockdev
 
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PIPELINE_SOURCE == 'push'
+
 build_stable:
   before_script:
     # Undo delangification present in the Fedora Docker images