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

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
    )