.multirust has moved back to home directory

[Diggsey]

The .multirust directory was moved to %localappdata% on windows, to follow platform conventions, and to avoid cluttering the home directory, but it has since been moved back.

I think there was also a plan to unify the .multirust and .cargo directories too?

[brson]

For consistency with where .cargo lives, .multirust is now in %USERPROFILE%\.multirust. If we want to change conventions here it needs to be done in both cargo and rustup.

There's no plan to unify .multirust and .cargo exactly. ~/.cargo/bin is where the rustup bins are installed - for consistency with other rust bins - but other than that rustup doesn't care about `~/.cargo.

[Diggsey]

OK, this should be tied to rust-lang/cargo#1976 then.