private static function cache_wp_files()
 {
     self::$cache_dir = sys_get_temp_dir() . '/wp-cli-test core-download-cache';
     if (is_readable(self::$cache_dir . '/wp-config-sample.php')) {
         return;
     }
     $cmd = Utils\esc_cmd('wp core download --force --path=%s', self::$cache_dir);
     Process::create($cmd, null, self::get_process_env_variables())->run_check();
 }