nixos-rebuild-ng: fix typos

# Use strings to avoid breaking standalone (e.g.: `python -m nixos_rebuild`)

# usage

EXECUTABLE = "@executable@"

# Use either `== "true"` if the default (e.g.: `python -m nixos_rebuld`) is

# Use either `== "true"` if the default (e.g.: `python -m nixos_rebuild`) is

# `False` or `!= "false"` if the default is `True`

WITH_NIX_2_18 = "@withNix218@" != "false"  # type: ignore

WITH_REEXEC = "@withReexec@" == "true"  # type: ignore

  # In case we want/need to evaluate packages or the assertions or whatever,

  # we want to have a linux system.

  # TODO: make the non-flake test use thise.

  # TODO: make the non-flake test use this.

  linuxSystem = lib.replaceStrings [ "darwin" ] [ "linux" ] stdenv.hostPlatform.system;

in