nix: don't install urbit-debug by default.
nix: don't install urbit-debug by default.

The urbit and urbit-debug derivations both produce an urbit-worker
binary, and they conflict.

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