Add reusable remote-builder NixOS module and builder system user.
Server (lazyworkhorse): dispatches aarch64-linux builds to uConsole
uConsole: dispatches x86_64-linux builds to server
The builder user uses the same SSH keypair on both hosts for
symmetric remote building. Generate the key with:
ssh-keygen -t ed25519 -f /etc/ssh/builder_key -N ""
Add the public key to lib/keys.nix (replace PLACEHOLDER).
