ci: add esp32 platform

.github/workflows/build.yml

@@ -19,6 +19,9 @@ jobs:
         uses: arduino/compile-sketches@v1
         with:
           fqbn: esp32:esp32:sensebox_mcu_esp32s2
+          platforms: |
+            - name: esp32:esp32
+              source-url: https://espressif.github.io/arduino-esp32/package_esp32_index.json
           sketch-paths: |
             - ./sensebox-bike-atrai
           libraries: |