Nix: improve buildIdris
Nix: improve buildIdris

- DRY
- idris2 is used as a compiler, should thus be in nativeBuildInputs

mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm