Actually add the cross.nix file.
Actually add the cross.nix file.

ngzax/urbit,jfranklin9000/urbit,jfranklin9000/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit