nixos-rebuild --upgrade that updates all nix-channels

default.nix

    nixos-rebuild = [(lib.hiPrio (pkgs.writeShellScriptBin "nixos-rebuild"
    ''
      REAL_ARGS=() ;
      while [[ $# -gt 0 ]] ; do
      if [[ --upgrade = $1 ]] ;
      then nix-channel --update ; shift ;
      fi ;
      REAL_ARGS+=("$1") ;
      shift ;
      done ;
      exec ${config.system.build.nixos-rebuild}/bin/nixos-rebuild ''${REAL_ARGS[@]}''
    ))];