Support new cmdline option --roots (#389)

* Add new field `Args::roots`
* Use `env::var_os` to fetch `CARGO_INSTALL_ROOTS`
   Previously, it uses `env::var`, which might reject valid path just
   because it is not utf-8 string.
* Update manifest if `CARGO_INSTALL_ROOT` is specified
* Add new fn `install_path::get_cargo_roots_path`
* Fix updating manifest: Use `cargo_roots` instead of default path
* Rm `helpers::statics::cargo_home`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

crates/binstalk/src/lib.rs

@@ -8,3 +8,4 @@ pub mod manifests;
 pub mod ops;
 
 pub use detect_targets::{get_desired_targets, DesiredTargets};
+pub use home;