Update flutter.yml

fix

Author: hoc081098

.github/workflows/flutter.yml

@@ -24,7 +24,7 @@ jobs:
 
       - uses: subosito/flutter-action@v1
         with:
-          channel: ${{ flutter-channel }}
+          channel: ${{ matrix.flutter-channel }}
 
       - name: Print Dart SDK version
         run: dart --version