Use major version ref of sketch compilation action

Previously, due to the lack of a release, the development versions of the sketch compilation actions were used. Using
release versions provides a more stable CI system for the ArduinoCore-mbed project.

Use of the major version ref will cause the workflow to benefit from ongoing development to the actions up until such
time as a new major release of an action is made, at which time we would need to evaluate whether any changes to the
workflow are required by the breaking change that triggered the major release before updating the major ref
(e.g., uses: 	`arduino/compile-sketches@v2`).

Author: per1234

.github/workflows/compile-examples.yml

@@ -40,7 +40,7 @@ jobs:
         uses: actions/checkout@v2
 
       - name: Compile examples
-        uses: arduino/compile-sketches@main
+        uses: arduino/compile-sketches@v1
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           fqbn: ${{ matrix.board.fqbn }}