Without this change, users that have both `initialHashedPassword` and
`hashedPassword` set will have `initialHashedPassword` take precedence,
but only for the first time `/etc/passwd` is generated. After that,
`hashedPassword` takes precedence. This is surprising behavior as it
would generally be expected for `hashedPassword` to win if both are set.
This wouldn't be a noticeable problem (and an assert could just be made
instead) if the users-groups module did not default the
`root.intialHashedPassword` value to `!`, to prevent login by default.
That means that users who set `root.hashedPassword` and use an ephemeral
rootfs (i.e. `/etc/passwd` is created every boot) are not able to log in
to the root account by default, unless they switch to a new generation
during the same boot (i.e. `/etc/passwd` already exists and
`hashedPassword` is used instead of `initialHashedPassword`) or they set
`root.initialHashedPassword = null` (which is unintuitive and seems
redundant).
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