`wp core download` picks up outer installs

Steps to reproduce:

```
$ mkdir /tmp/test-wp-download && cd /tmp/test-wp-download
$ wp core download
Downloading latest WordPress (en_US)...
Success: WordPress downloaded.
$ mkdir inner && cd inner
$ wp core download
Error: WordPress files seem to already be present here.
```

Original report: https://twitter.com/aaroneaton/status/421469536647266305