[rustbuild] ignores the --build flag

[nagisa]

build being commented-out in config.toml and running

python x.py test --stage 2 --build=i686-unknown-linux-gnu

starts downloading the x86_64 compiler… and then tries to execute i686 compiler which fails with a terrible error of

failed to execute command: No such file or directory (os error 2)

[Mark-Simulacrum]

Yeah, still a problem.

[Mark-Simulacrum]

Should be fixed by #43384.