While doing some digging, I discovered that it is possible for a crate to define environment variables in its subprocesses including its dependent crates, and apparently this has been stable for a number of years.
However, I can't find any mention of this in The Book, either in the section talking about the manifest format or how environment variables work - the latter section only talks about the ones Cargo itself sets up.
I just wanted to check that this is really not there and I'm not just missing something before I started looking into how to submit a correction