ci: use detached CI pipelines

See [1], required to allow contributors to trigger CI pipelines
for MRs. Example failure can be found at [2].

[1]: https://gitlab.freedesktop.org/freedesktop/freedesktop/-/issues/540#what-it-means-for-me-a-maintainer-of-a-project-part-of-gitlabfreedesktoporg
[2]: https://gitlab.freedesktop.org/emersion/libdisplay-info/-/jobs/44443608

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

.gitlab-ci.yml

@@ -13,6 +13,11 @@ variables:
   FDO_UPSTREAM_REPO: 'emersion/libdisplay-info'
   FDO_DISTRIBUTION_TAG: '2023-01-31.0'
 
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PIPELINE_SOURCE == 'push'
+
 stages:
   - "Contribution checks"
   - "Prepare container"