default.nix

{ pkgs, ... }:
let
  wrappedHello = pkgs.symlinkJoin {
    name = "hello";
    paths = [ pkgs.hello ];
    nativeBuildInputs = [ pkgs.makeWrapper ];
    postBuild = ''
      wrapProgram $out/bin/hello \
        --add-flags "-t"
    '';
  };
in {
  environment.systemPackages = [ wrappedHello ];
}