Fix Flutter setup in GitHub action

Author: matthew-carroll

.github/workflows/pr_validation.yaml

@@ -16,7 +16,7 @@ jobs:
       - name: Set up Flutter
         uses: subosito/flutter-action@v2
         with:
-          flutter-version: stable
+          channel: stable
           architecture: x64
 
       - name: Install Android SDK and launch emulator