private static function cache_wp_files() {
		$wp_version        = getenv( 'WP_VERSION' );
		$wp_version_suffix = ( false !== $wp_version ) ? "-$wp_version" : '';
		self::$cache_dir   = sys_get_temp_dir() . '/wp-cli-test-core-download-cache' . $wp_version_suffix;

		if ( is_readable( self::$cache_dir . '/wp-config-sample.php' ) ) {
			return;
		}

		$cmd = Utils\esc_cmd( 'wp core download --force --path=%s', self::$cache_dir );
		if ( $wp_version ) {
			$cmd .= Utils\esc_cmd( ' --version=%s', $wp_version );
		}
		Process::create( $cmd, null, self::get_process_env_variables() )->run_check();
	}