.github/workflows: use pip caching

See: https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#caching-packages

.github/workflows/python-app.yml

@@ -20,6 +20,7 @@ jobs:
       uses: actions/setup-python@v4
       with:
         python-version: '3.10'
+        cache: 'pip'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip