`RUSTUP_WINDOWS_PATH_ADD_BIN`, as mentioned in #3703, isn't documented on the website: <https://rust-lang.github.io/rustup/environment-variables.html>. I think it should be documented, however, because it's useful when writing a program that links to `rustc_driver.dll`. (See https://github.com/TheBevyFlock/bevy_cli/issues/267 for context.)