Warning about root user options in nixpkgs 24.05

[mickours]

When starting a build with nxc 0.6 and nixpkgs 24.05 this message appears multiple time:

trace: warning: The user 'root' has multiple of the options
`hashedPassword`, `password`, `hashedPasswordFile`, `initialPassword`
& `initialHashedPassword` set to a non-null value.
The options silently discard others by the order of precedence
given above which can lead to surprising results. To resolve this warning,
set at most one of the options above to a non-`null` value.

We should select the right option.