Fix YAML syntax for branches in workflow

Author: mithun50

.github/workflows/flutter-build.yml

@@ -2,7 +2,7 @@ name: Build OpenClaw Apps
 
 on:
   push:
-    branches: *
+    branches: ['*']
     paths:
       - 'flutter_app/**'
       - 'scripts/fetch-proot-binaries.sh'