try to check out head

.github/workflows/check-version.yml

@@ -10,6 +10,7 @@ jobs:
     steps:
       - uses: actions/checkout@v1
         with:
+          ref: ${{ github.event.pull_request.head.sha }}
           fetch-depth: 0
 
       - name: Set OLD_VERSION from etc/config.xml if merge would modify it