[Nix-dev] What am I doing wrong with ssh-substitutor-hosts?

[Jeffrey David Johnson]

I've got two computers with nixos, and I can ssh between them without a
password.  But if I do a nixos-rebuild with `--option ssh-substitutor-hosts
<hostname>` they still just download everything from cache.nixos.org.  I think
I have the sshServe set up in my configuration.nix:

# nixServeKeys are loaded from another file keys.nix,
# and are all the same ones used for regular ssh access
nix.sshServe = { enable = true; keys = nixServeKeys; };

Is there a setting I'm missing somewhere?
Jeff