ci: Add workflow rules to create merge request pipelines

This should make Weblate MRs run again.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/827>

.gitlab-ci.yml

@@ -10,6 +10,15 @@
 # there doesn't seem to be significant value to splitting the stages at the
 # moment.
 
+# Create merge request pipelines for open merge requests, branch pipelines
+# otherwise. This allows MRs for new users to run CI.
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
+    - if: $CI_COMMIT_BRANCH
+
 stages:
   - container
   - build