flutter pub get慢，需要更改镜像源
| 变量 | 值 |
|---|---|
| PUB_HOSTED_URL | https://pub.flutter-io.cn |
| FLUTTER_STORAGE_BASE_URL | https://storage.flutter-io.cn |
参考：https://docs.flutter.dev/community/china#configuring-flutter-to-use-a-mirror-site
| 变量 | 值 |
|---|---|
| PUB_HOSTED_URL | https://pub.flutter-io.cn |
| FLUTTER_STORAGE_BASE_URL | https://storage.flutter-io.cn |
参考：https://docs.flutter.dev/community/china#configuring-flutter-to-use-a-mirror-site
