x.py does not download boostrap compiler if --build differs from default `build`

[nagisa]

With build in config.toml being commented out, running (on x86_64) following command fails:

$ python x.py test --host=i686-unknown-linux-gnu --stage=2 --target=i686-unknown-linux-gnu --build=i686-unknown-linux-gnu
    Finished debug [unoptimized] target(s) in 0.0 secs


failed to execute command: "./build/i686-unknown-linux-gnu/stage0/bin/rustc" "--version" "--verbose"
error: No such file or directory (os error 2)

[alexcrichton]

This can likely be fixed by updating this function, which seems to just not have the proper support yet

[Mark-Simulacrum]

Should be fixed by #43384.