Use the latest ArduinoCore-samd from master, not the latest released version

Author: aentinger

.github/workflows/compile-examples.yml

@@ -25,6 +25,9 @@ jobs:
     strategy:
       fail-fast: false
 
+    platforms:
+      - source-url: https://github.com/arduino/ArduinoCore-samd/archive/master.zip
+
       matrix:
         board: [
           {"fqbn": "arduino:samd:mkr1000", "type": "wifi"},