rust: don't build out of tree

[ognevny]

this is not supported upstream, but a workaround was used. set build-dir in config.toml, make dest dirs names contain $MSYSTEM to match previous behavior

asking @filnet as a workaround's author is it ok to do so

[filnet]

asking @filnet as a workaround's author is it ok to do so

I am not sure that I understand your question.
But note that I have currently lost interest in Rust (might come back some day though).

[ognevny]

I meant will be fine to drop workaround for building outside of rustc src and build inside of it?