make it working with nightly, set RUST_ROOT to /usr by default

[Fiedzia]

No description provided.

[noboruma]

Does that actually make sense to locate cargo with a RUST_ROOT variable?
I would argue that it is user responsibility to make sure cargo is located on the PATH.
(Personally my cargo is located in /usr/local/bin, while my rustc is in ~/usr/bin).
I would rather suggest to simply get rid of RUST_ROOT.