document environment variables in the unstable book

[jyn514]

Location

https://doc.rust-lang.org/nightly/unstable-book/

Summary

right now, https://doc.rust-lang.org/nightly/unstable-book/compiler-flags.html documents unstable compiler flags. but to my knowledge, there is no equivalent place to document compiler environment variables (such as RUSTC_ICE). currently we hack in RUSTC_BOOTSTRAP and RUSTC_OVERRIDE_VERSION_STRING to the flags section. i think it would be nice to have a dedicated section, encouraging people to document the env vars.