nixos/manual: prompt to set root password has changed
nixos/manual: prompt to set root password has changed

The manual mentions in
https://nixos.org/manual/nixos/stable/index.html#sec-installation-installing:
```
setting root password...
Enter new UNIX password: ***
Retype new UNIX password: ***
```

However, in the terminal I got:
```
setting root password...
New password: ***
Retype new password: ***
```

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