systemd: disable utmp on musl (#464535)

  # adds python to closure which is too much by default

  withUkify ? false,

  withUserDb ? true,

  withUtmp ? true,

  # utmp does not exist on musl, so it would be implicitly disabled

  # It is important to document the lack of utmp in nix,

  # otherwise the condition for systemd-update-utmp.service will

  # attempt to load a service which does not exist, resulting in errors.

  withUtmp ? !stdenv.hostPlatform.isMusl,

  withVmspawn ? true,

  # kernel-install shouldn't usually be used on NixOS, but can be useful, e.g. for

  # building disk images for non-NixOS systems. To save users from trying to use it