Only check compilers prior to merge

.github/workflows/check-compilers.yml

@@ -1,6 +1,14 @@
 name: Check compilers
 
-on: [push, pull_request]
+on:
+  # run every day at 06:00 UTC
+  schedule:
+    - cron: '0 6 * * *'
+  # when triggered manually
+  workflow_dispatch:
+  # when auto merge is enabled (hack to make sure it's run before merging)
+  pull_request:
+    types: [auto_merge_enabled]
 
 # Cancel "duplicated" workflows triggered by pushes to internal
 # branches with associated PRs.