Add support for [env] in rust-toolchain.toml

[jonhoo]

Fixes #2883.

Should probably not be merged until the open questions from #2883 are resolved, and notably the implication of the discussion in https://github.com/rust-lang/rustup/issues/2793 is taken into account.

[kinnison]

Thanks Jon, I've marked this as draft for now because until we're confident the decisions are right, I don't want to risk merging this. I'll try and get to code review at some point soonish though, I promise.