系统环境变量设置
FLUTTER_GIT_URL=https://gitee.com/mirrors/Flutter.git
PUB_HOSTED_URL=https://pub.flutter-io.cn
FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn
确保可以ping通上面的url，这是下载的关键
否则，需要参考：
flutter国内镜像和下载地址集合_flutter 国内镜像-CSDN博客
克隆代码
本人这里使用了gitee的加速源
git clone -b main https://gitee.com/mirrors/Flutter.git
检查安装情况
flutter doctor