Print out env vars related to Rust on (sufficiently verbose) rustc invocations.
Print out env vars related to Rust on (sufficiently verbose) rustc invocations.

Fix issue 38686.

(update: placated tidy.)

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust