Enable Pip caching in when running GitHub Actions

Re spine-tools/Spine-Toolbox#2642

.github/workflows/run_unit_tests.yml

@@ -24,6 +24,10 @@ jobs:
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
+        cache: pip
+        cache-dependency-path: |
+          requirements.txt
+          pyproject.toml
     - name: Display Python version
       run:
          python -c "import sys; print(sys.version)"