github: cache venv when running tests

.github/workflows/tests.yaml

@@ -27,6 +27,13 @@ jobs:
         uses: actions/setup-python@v4
         with:
           python-version: "3.10"
+      - name: Cache virtual environment
+        uses: actions/cache@v3
+        with:
+          # We are hashing dev-requirements.txt and test-requirements.txt which
+          # contain all dependencies needed to run the tests.
+          key: venv-${{ runner.os }}-${{ steps.setup_python.outputs.python-version}}-${{ hashFiles('dev-requirements.txt') }}-${{ hashFiles('test-requirements.txt') }}
+          path: .venv
       - name: Install system packages
         id: install_system_packages
         run: |