nixos-configuration/shared/base/modules/security/ssh.nix

# /nix/store/<hash>/etc/ssh/ssh_config & /nix/store/<hash>/etc/ssh/authorized_keys
{
  lib,
  systemConfig,
  systems,
  ...
}:
{
  programs.ssh.knownHosts = builtins.mapAttrs (hostName: system: {
    publicKey = system.ssh.publicKey;
  }) systems;
  users.users.${systemConfig.username}.openssh.authorizedKeys.keys = (
    lib.mapAttrsToList (_hostName: system: system.ssh.publicKey) systems
  );
}