Fix CI workflow

Author: b-per

.github/workflows/ci.yml

@@ -30,6 +30,8 @@ jobs:
     steps:
     - name: Checkout code
       uses: actions/checkout@v4
+      with:
+          ref: ${{ github.event.pull_request.head.sha }}
 
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v5