Rollup merge of #141526 - jyn514:env-vars, r=petrochenkov

add a dedicated section for compiler environment variables in the unstable book

rendered: ![image](https://github.com/user-attachments/assets/dcb6e2a3-2e2d-43bb-8f45-79715705bf45)

fixes rust-lang/rust#141523

Author: jieyouxu