chore: support merge queues in CI

Add the merge_group event to the CI workflows so the checks also run against
the merge commit built by the merge queue. Without it the required checks
never report on a queued pull request and the queue stalls.

Author: NSeydoux

.github/workflows/ci.yml

@@ -1,6 +1,7 @@
 name: CI
 
-on: [push]
+# Run the required tests on the merge queue.
+on: [push, merge_group]
 
 env:
   CI: true