ci: add caching for test job

.github/workflows/ci.yml

@@ -37,6 +37,20 @@ jobs:
         uses: actions/checkout@v2
       - name: Install GCC compiler
         run: sudo apt-get install build-essential
+      - name: Cache pip
+        uses: actions/cache@v2
+        with:
+          path: ~/.cache/pip
+          key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-
+      - name: Cache PlatformIO
+        uses: actions/cache@v2
+        with:
+          path: ~/.platformio
+          key: ${{ runner.os }}-${{ hashFiles('**/lockfiles') }}
+      - name: Use Python
+        uses: actions/setup-python@v2
       - name: Install PlatformIO Core
         run: pip install -U platformio
       - name: Run tests
@@ -49,7 +63,7 @@ jobs:
           rm .pio/build/native/test/tmp_pio_test_transport.*
           gcovr -r . -e .pio/libdeps -e test -s
           gcovr -r . -e .pio/libdeps -e test --html-details -o code-coverage/coverage.html
-      - name: Archive code coverage
+      - name: Upload code coverage as artifact
         uses: actions/upload-artifact@v2
         with:
           name: code-coverage