Allow the unstable --error-format flag to be changed from an environment variable

[trixnz]

Continuing discussion from https://github.com/saviorisdead/RustyCode/issues/146:

I've been working towards adding support for the new JSON error format in RustyCode, however I've come across a bit of an issue that slows down iteration times for users. If the IDE extension issues cargo commands with the RUSTFLAGS environment variable set to -Z unstable-options --error-format=json then, as expected, we get JSON error messages. If the user was to then manually compile their project from a terminal with cargo build, the fingerprint changes because RUSTFLAGS no longer matches the last compilation and a full rebuild occurs. This also occurs the next time the user builds or saves the file from the IDE for the same reason.

For some (clippy, check) subcommands, this can be avoided by passing the arguments in directly. Some, however, can't facilitate this (bench, test, build, etc.).

I propose a new environment variable called RUST_ERROR_FORMAT that only changes the format that errors are printed with without changing the fingerprint. My only concern is that because this is an unstable feature right now, such an environment variable would bypass -Z unstable-options.

[trixnz]

Closed in favour of moving this into Cargo now that JSON output is stabilised: rust-lang/cargo#2982