Cache Python & PlatformIO dependencies (#5822)

.github/actions/build-variant/action.yml

@@ -68,6 +68,12 @@ runs:
           sed -i '/DDEBUG_HEAP/d' ${INI_FILE}
         done
 
+    - name: PlatformIO ${{ inputs.arch }} download cache
+      uses: actions/cache@v4
+      with:
+        path: ~/.platformio/.cache
+        key: pio-cache-${{ inputs.arch }}-${{ hashFiles('.github/actions/**', '**.ini') }}
+
     - name: Build ${{ inputs.board }}
       shell: bash
       run: ${{ inputs.build-script-path }} ${{ inputs.board }}