Core download command failing in VVV

/cc @jeremyfelt

I'm trying to get `wp core download` working in a directory in a [VVV](https://github.com/10up/varying-vagrant-vagrants) virtual machine. When I run the command I get:

```
root@vvv:/srv/www/vvv-demo-1/htdocs# wp core download
Downloading latest WordPress (en_US)...

Warning: rename(): The first argument to copy() function cannot be a directory in /srv/www/wp-cli/php/commands/core.php on line 114

Warning: rename(/tmp/wp_52703b5c866dc/wordpress/wp-content,/srv/www/vvv-demo-1/htdocs//wp-content): Invalid cross-device link in /srv/www/wp-cli/php/commands/core.php on line 114

Warning: rename(): The first argument to copy() function cannot be a directory in /srv/www/wp-cli/php/commands/core.php on line 114

Warning: rename(/tmp/wp_52703b5c866dc/wordpress/wp-includes,/srv/www/vvv-demo-1/htdocs//wp-includes): Invalid cross-device link in /srv/www/wp-cli/php/commands/core.php on line 114

Warning: rename(): The first argument to copy() function cannot be a directory in /srv/www/wp-cli/php/commands/core.php on line 114

Warning: rename(/tmp/wp_52703b5c866dc/wordpress/wp-admin,/srv/www/vvv-demo-1/htdocs//wp-admin): Invalid cross-device link in /srv/www/wp-cli/php/commands/core.php on line 114

Warning: rmdir(/tmp/wp_52703b5c866dc/wordpress): Directory not empty in /srv/www/wp-cli/php/commands/core.php on line 116

Warning: rmdir(/tmp/wp_52703b5c866dc): Directory not empty in /srv/www/wp-cli/php/commands/core.php on line 92
```

This looks a bit like [PHP bug 54097](https://bugs.php.net/bug.php?id=54097).