Pin to last compatible action version in sketch compile workflow (#20)

This library requires a modified "arduino_threads" variant of Arduino CLI. Since the time that variant was made, there
was a breaking change to the key names of Arduino CLI's JSON format output. The `arduino/compile-sketches` action uses
some of the affected keys and so had to be updated to use the new keys. A translation system was put in place to provide
backwards compatibility. However, this translation system requires that the Arduino CLI version specified via the
action's `cli-version` input be semver-compliant so that it can determine whether it came before or after the breaking
change.

The "arduino_threads" variant of Arduino CLI has the fatal combination of the old keys and non-semver version, meaning it is not compatible with versions of the `arduino/compile-sketches` action that came after the fix for the key name changes:

```
Installing platform from path: extras/ArduinoCore-mbed
Traceback (most recent call last):
  File "/home/runner/work/_actions/arduino/compile-sketches/main/compilesketches/compilesketches.py", line 1717, in <module>
    main()  # pragma: no cover
  File "/home/runner/work/_actions/arduino/compile-sketches/main/compilesketches/compilesketches.py", line 56, in main
    compile_sketches.compile_sketches()
  File "/home/runner/work/_actions/arduino/compile-sketches/main/compilesketches/compilesketches.py", line 198, in compile_sketches
    self.install_platforms()
  File "/home/runner/work/_actions/arduino/compile-sketches/main/compilesketches/compilesketches.py", line 311, in install_platforms
    self.install_platforms_from_path(platform_list=platform_list.path)
  File "/home/runner/work/_actions/arduino/compile-sketches/main/compilesketches/compilesketches.py", line 493, in install_platforms_from_path
    platform_installation_path = self.get_platform_installation_path(platform=platform)
  File "/home/runner/work/_actions/arduino/compile-sketches/main/compilesketches/compilesketches.py", line 532, in get_platform_installation_path
    if installed_platform[self.cli_json_key("core list", "ID")] == platform[self.dependency_name_key]:
KeyError: 'id'
```

The workaround is to pin version of the `arduino/compile-sketches` action used in the "Compile Examples" workflow to the
last compatible ref. Fortunately, that is quite recent in the action's revision history and none of the changes that came
since should be relevant for this application.

Author: per1234

.github/workflows/compile-examples.yml

@@ -72,7 +72,9 @@ jobs:
           mv "${{ env.ARDUINOCORE_API_STAGING_PATH }}/api" "${{ env.ARDUINOCORE_MBED_STAGING_PATH }}/cores/arduino"
 
       - name: Compile examples
-        uses: arduino/compile-sketches@main
+        # Due to using obsolete JSON output keys and a non-semver version, the modified "arduino_threads" version of
+        # Arduino CLI is not compatible with any version of `arduino/compile-sketches` after this ref, thus the pin:
+        uses: arduino/compile-sketches@15d79dc244a7a19b8129a83d4d34c6ba44859f01
         with:
           cli-version: 'arduino_threads'
           fqbn: ${{ matrix.fqbn }}