Cache pre-commit in GitHib Actions (#7400)

See https://pre-commit.com/#github-actions-example

.github/workflows/python-linters.yml

@@ -26,6 +26,12 @@ jobs:
       uses: actions/setup-python@v1
       with:
         python-version: ${{ matrix.env.PYTHON_VERSION || 3.7 }}
+    - name: set PY
+      run: echo "::set-env name=PY::$(python -VV | sha256sum | cut -d' ' -f1)"
+    - uses: actions/cache@v1
+      with:
+        path: ~/.cache/pre-commit
+        key: pre-commit|${{ env.PY }}|${{ hashFiles('.pre-commit-config.yaml') }}
     - name: Pre-configure global Git settings
       run: >-
         tools/travis/setup.sh