ci: Fix up workflow rules for MR vs. branch pipelines

.gitlab-ci.yml

@@ -10,6 +10,16 @@
 # as part of the build stage as 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, and prevents duplicate
+# pipelines for branches with open MRs.
+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