Rust更改源

更改 ~/.cargo/config 文件为以下内容，设置源为中科大的：

[source.crates-io]
registry = "https://github.com/rust-lang/crates.io-index"
replace-with = 'ustc'
[source.ustc]
registry = "git://mirrors.ustc.edu.cn/crates.io-index"