nixos/collectd: add missing group

While upgrading my NixOS system I was greeted by this error:

  error:
  Failed assertions:
  - users.users.collectd.group is unset. This used to default to
  nogroup, but this is unsafe. For example you can create a group
  for this user with:
  users.users.collectd.group = "collectd";
  users.groups.collectd = {};

Let's fix it.

nixos/modules/services/monitoring/collectd.nix

@@ -132,7 +132,12 @@ in {
     users.users = optionalAttrs (cfg.user == "collectd") {
       collectd = {
         isSystemUser = true;
+        group = "collectd";
       };
     };
+
+    users.groups = optionalAttrs (cfg.user == "collectd") {
+      collectd = {};
+    };
   };
 }