CI: Run 'Verify formatting' only on stable sdk

Author: Bungeefan

.github/workflows/dart.yml

@@ -33,6 +33,7 @@ jobs:
 
       # Uncomment this step to verify the use of 'dart format' on each commit.
       - name: Verify formatting
+        if: matrix.sdk == 'stable'
         run: dart format --output=none --set-exit-if-changed .
 
       # Consider passing '--fatal-infos' for slightly stricter analysis.