ci: use MR pipelines

This runs CI jobs in the context of the parent project, unlocking
merge request env vars.

See https://docs.gitlab.com/ci/pipelines/merge_request_pipelines/#run-pipelines-in-the-parent-project

.gitlab-ci.yml

@@ -1,4 +1,10 @@
 include: https://gitlab.freedesktop.org/emersion/dalligi/-/raw/master/templates/multi.yml
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
+    - if: $CI_COMMIT_BRANCH
 alpine:
   extends: .dalligi
   pages: true