Suggested by Mark H. Weaver.

* nix/nix-daemon/guix-daemon.cc (GUIX_OPT_NO_SUBSTITUTES): New macro.
  (options): Add `--no-substitutes'.
  (parse_opt): Add `GUIX_OPT_NO_SUBSTITUTES' case.
  (main): Leave `settings.substituters' empty when
  `settings.useSubstitutes' is false.