Include base platform because it wants it.

.github/workflows/main.yml

@@ -8,6 +8,7 @@ jobs:
       - uses: arduino/compile-sketches@v1.1.0
         with:
           platforms: |
+            - name: arduino:avr
             - source-url: "https://adafruit.github.io/arduino-board-index/package_adafruit_index.json"
               name: "adafruit:samd"
           sketch-paths: |