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Rustbuild passes --message-format=json to Cargo to learn about the
dependencies for a given build, which then makes Cargo steal the
stderr/stdout for the compiler process, leading to non colorful output.
To avoid this, detection of stderr being a tty is added to rustbuild,
and an environment variable is used to communicate with the rustc shim.
Fixes #42801.
r? @alexcrichton