Add os/arch options to .nimble?
When compiling for an embedded platform like raspberry pi, microprocessors, nintendo switch, or graphing calculators, you usually never want to build for your host machine but rather a specific os/arch tuple, and it can be annoying to always have to pass `--os:_ --arch:_` to `nimble build`. Maybe there could be `os` and `arch` options in `.nimble`, similar to `backend`?