Enable pip caching for GitHub Actions

Enable pip caching in order to avoid repeatedly rebuilding all
the PyPy wheels for source packages.  This is inspired by discussion
in psf/requests#6496 and it may help with
the PyPy CI failures.

.github/workflows/ci.yml

@@ -21,6 +21,7 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+        cache: pip
     - name: Install dependencies
       run: |
         python -m pip install .