nixos: add uid/gid for munin

To be compatible with eb2f44c18cb6d300e965308547d8a4dea110f519 (Generate
/etc/passwd and /etc/group at build time). Without this you'll get this:

  $ nixos-rebuild build
  [...]
  user-thrown exception: The option `users.extraGroups.unnamed-9.1.gid' is used but not defined.