Export `CARGO_PKG_EDITION` variable to build environment

[nyurik]

Problem

Some tools like bindgen may need to know the Rust language edition of the root crate, especially when executing as part of build.rs. For example, bindgen needs to know if the -sys package uses edition 2021 (so it can use C-string literals), or older (in which case it must generate some unsafe code with the same literals). Requiring the user to always pass in bindgen args that match their environment is less than ideal if the default could be computed on the fly using environment variables.

See rust-lang/rust-bindgen#3000

Proposed Solution

Publish CARGO_PKG_EDITION or CARGO_PKG_RUST_EDITION or CARGO_PKG_LANG_EDITION or ... (naming tbd)

Notes

No response

[weihanglo]

Probably a dup of #6408?