Fix GItHub actions to checkout proper branch

.github/workflows/build-and-check-python-package.yml

@@ -27,6 +27,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Set up Python
         uses: actions/setup-python@v5

.github/workflows/ci.yml

@@ -15,6 +15,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
 
     - name: Set up Python 3.10
       uses: actions/setup-python@v5