Trigger CI on GitHub's merge queue

This doesn't enable the merge queue, but is a prerequisite. See
<https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#triggering-merge-group-checks-with-github-actions>
for details.

.github/workflows/ci.yml

@@ -1,7 +1,7 @@
 ---
 name: CI
 
-on: [push, pull_request]
+on: [merge_group, push, pull_request]
 
 jobs:
   checks:

.github/workflows/reprotest.yml

@@ -2,6 +2,7 @@
 name: CI
 
 on:
+  merge_group:
   push:
   pull_request:
   schedule: