CI: enable pip cache

Use actions/setup-python's pip cache support on workflow jobs that install
dependencies.

Author: medmunds

.github/workflows/integration-test.yml

@@ -59,6 +59,7 @@ jobs:
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.config.python }}
+          cache: "pip"
       - name: Install tox
         run: |
           set -x

.github/workflows/test.yml

@@ -53,6 +53,7 @@ jobs:
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.tox.python.spec }}
+          cache: "pip"
       - name: Setup default Python
         # Change default Python version back to something consistent
         # for installing/running tox