`nixos-enter passwd` fails with "command not found" after finix is installed

[deathbymanatee]

to reproduce: follow the install guide from examples repository and install finix with nixos-install --root /path/to/root --flake /path/to/flake#finix. attempt to set a user password with nixos-enter --root /mnt -c 'passwd alice'. observe failure.

[aanderse]

oh... we should modify the documentation to explicitly add mkpasswd to environment.systemPackages