Check if `~/.cargo/bin` already on PATH

[chriskrycho]

During first setup, especially for users who have used multirust or rustup in the past, it's possible they'll already have ~/.cargo/bin in their path configuration already. (It's already in my .profile, for example.) Instead of always adding it to the end of the user's profiles, maybe check the PATH for it first?

[brson]

I think I like this idea, though it's a little clever and I'm wary of guessing the user's intent incorrectly. I guess we would check for whether PATH contains the substring ".cargo/bin" or ".cargo\bin".

This does seem like a niche case though - most people installing rustup should not have a preexisting install of Cargo from some other source.

There's also now an interactive option to opt out of modifying PATH for those that know they don't need it.