ci: add pip cache to GHA pr.yaml

Author: ketozhang

Diff for: .github/workflows/pr.yml

@@ -29,6 +29,7 @@ jobs:
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
+        cache: 'pip'
     - name: Install Python dependencies
       run: |
         python -m pip install --upgrade pip