fix: move ref from merge commit to head

.github/workflows/linting.yml

@@ -17,6 +17,7 @@ jobs:
       - uses: actions/checkout@v2
         with:
           fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha }}
       - uses: actions/setup-python@v2
         with:
           cache: 'pip'