drush_is_local_host() doesn't seem to get run for shell alias commands that start with !

I have the following in my sites/all/drush/drushrc.php:
```php
$root = str_replace('/drush', '', dirname(__FILE__));
$options['shell-aliases']['build-theme'] = '!cd ' . $root . '/themes/custom/mytheme && npm install && bundle install && grunt --env=production';
```

And my Drush alias is defined as the following:
```php
$aliases['not-really-remote'] = array(
  'root' => '/var/www/local.mysite.com/',
  'uri' => 'local.mysite.com',
  'remote-host' => 'Corsair250D', // This matches my current *local* computer's host name.
);
```

When I run drush @not-really-remote build-theme I get a SSH prompt, which is very unexpected:
```
davereid@Corsair250D$ drush @not-really-remote build-theme
The authenticity of host 'corsair250d (127.0.1.1)' can't be established.
ECDSA key fingerprint is 0a:e9:83:1c:59:0e:e1:b9:a1:2d:43:e8:83:77:e9:6f.
Are you sure you want to continue connecting (yes/no)?
```