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.

1 files changed, 2 insertions, 0 deletions

diff --git a/nixos/modules/services/monitoring/munin.nix b/nixos/modules/services/monitoring/munin.nix
index 153f4942902..39c4fb1aefa 100644
--- a/nixos/modules/services/monitoring/munin.nix
+++ b/nixos/modules/services/monitoring/munin.nix
@@ -173,10 +173,12 @@ in
       name = "munin";
       description = "Munin monitoring user";
       group = "munin";
+      uid = config.ids.uids.munin;
     }];
 
     users.extraGroups = [{
       name = "munin";
+      gid = config.ids.gids.munin;
     }];
 
   }) (mkIf nodeCfg.enable {