[Nix-dev] Cross development for STM32

[stewart mackenzie]

I'd really like to see a simple nix way to retarget derivations.

So maybe `buildRustPackageForAndroid` or `buildRustPackageForSTM32`.

or maybe

buildRustPackage rec {
   name = "habitat-${version}";
   version = "0.8.0";
   target = "android";
   ...
}

kr/sjm