chore：fix flutter version 1.20.1 build error

This application cannot tree shake icons fonts. It has non-constant instances of IconData at the following locations:

Author: hackycy

Diff for: .github/workflows/build.yml

@@ -26,7 +26,7 @@ jobs:
           channel: 'stable' # or: 'dev' or 'beta'
       - run: flutter pub get
       - run: flutter test
-      - run: flutter build apk --release
+      - run: flutter build apk --release --no-tree-shake-icons
       - uses: actions/upload-artifact@v2
         with:
           path: build/app/outputs/apk/release/*.apk