use the "home" crate to get CARGO_HOME location

Apparently rustup has its [own implementation](https://github.com/rust-lang/rustup.rs/blob/master/src/utils/utils.rs#L548) of getting the `${CARGO_HOME}` location however it could just use the official `home` crate that `cargo` is using to do the same thing:

https://crates.io/crates/home
https://docs.rs/home/0.3.4/home/fn.cargo_home.html
