chore: migrate to checkout v4 and use head instead of master

Author: saschb2b

.github/workflows/pr.yml

@@ -12,9 +12,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
-          fetch-depth: 0
+          fetch-depth: 2
+      - run: git checkout HEAD^
       - name: Setup Node.js
         uses: actions/setup-node@v2
         with: