Fix path handling in core download - #327

It fails because of this :

        if ( false === strpos( $command, '--path' ) ) {
            $command .= \WP_CLI\Utils\assoc_args_to_str( array(
                'path' => $this->install_dir
            ) );
        }

e.g : when the command has a pipe like wp user list | wc -l, the --path is not at the right place : wp user list | wc -l --path='/tmp/wp-cli-test-512dd489ed31d3.92337052'

Hm... guess I'll have to make the injection procedure smarter. Thanks for investigating.