Merge pull request #132 from google/setup-python-cache-pip

ci: use new cache directive in setup-python

.github/workflows/ci.yml

@@ -10,6 +10,7 @@ jobs:
       - uses: actions/setup-python@v2.3.0
         with:
           python-version: '3.7'
+          cache: 'pip'
       - name: install dependencies
         run: |
           sudo apt-get update -y