Update subosito/flutter-action action to v2

Author: renovate-bot

.github/workflows/build.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: actions/setup-java@v1
         with:
           java-version: '12.x'
-      - uses: subosito/flutter-action@v1.5.3
+      - uses: subosito/flutter-action@v2.2.1
         with:
           flutter-version: '2.5.1'
           channel: 'stable'