more guessing

But, this **should** work, per
arduino/compile-sketches#73

Author: henrygab

.github/workflows/arduino.yml

@@ -134,7 +134,7 @@ jobs:
         uses: arduino/compile-sketches@1edb1fde2dd10524d345c853e888f487459667aa # v1.0.1
         with:
           fqbn: ${{ matrix.fqbn }}
-          platforms:
+          platforms: |
           - source-url: http://arduino.esp8266.com/stable/package_esp8266com_index.json
             name: esp8266:esp8266
           - source-url: https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json