nixos-config/users/root/default.nix

{ pkgs, ... }: {
  users.users.root = {
    openssh.authorizedKeys.keyFiles = [
      (pkgs.fetchurl {
        url = "https://github.com/eboskma.keys";
        sha256 = "uwK4FSLSHiwCJU9U7RBFHIoCmr7uUQLM0JM1u0bi4xo=";
      })
    ];
  };
}