Also run conflict detection job on merge queues

Merge queues also require CI jobs to succeed, but they don't get
triggered by the `pull_request` trigger:
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

Our other GitHub Actions workflows appear to also have `push` as
a trigger, and therefore do not need this change.

Author: Vinnl

.github/workflows/conflicts.yml

@@ -5,6 +5,8 @@ permissions: {}
 on:
   pull_request:
     branches: [ main, localization ]
+  merge_group:
+    types: [checks_requested]
 
 jobs:
   detect-unresolved-conflicts: