docs: def set_cache(config_fpath, cache_dir):
      """Write the cache directory to the dtool config file.
  
      :param config_fpath: path to the dtool config file
      :param cache_dir: the path to the dtool cache direcotory
      """
      cache_dir = os.path.abspath(cache_dir)
      return write_config_value_to_file(
          CACHE_DIRECTORY_KEY,
          cache_dir,
          config_fpath
      )