chore: use head for check code job instead of master

Author: saschb2b

.github/workflows/pr.yml

@@ -15,6 +15,7 @@ jobs:
         uses: actions/checkout@v2
         with:
           fetch-depth: 0
+          ref: ${{ github.head_ref }}
       - name: Setup Node.js
         uses: actions/setup-node@v2
         with: