Checkout before changed-files action

Because the action cannot use GH API for merge_group events

Author: reakaleek

.github/workflows/preview-build.yml

@@ -40,6 +40,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha || github.ref }}
+          persist-credentials: false
+
       - name: Get changed files
         if: startsWith(github.event_name, 'pull_request') || github.event_name == 'merge_group'
         id: check-files