nixos/tvheadend: define group, fix eval after #133166

nixos/modules/misc/ids.nix

@@ -231,7 +231,7 @@ in
       skydns = 197;
       # ripple-rest = 198; # unused, removed 2017-08-12
       # nix-serve = 199; # unused, removed 2020-12-12
-      tvheadend = 200;
+      #tvheadend = 200; # dynamically allocated as of 2021-09-18
       uwsgi = 201;
       gitit = 202;
       riemanntools = 203;

nixos/modules/services/networking/tvheadend.nix

@@ -29,8 +29,10 @@ in
       description = "Tvheadend Service user";
       home        = "/var/lib/tvheadend";
       createHome  = true;
-      uid         = config.ids.uids.tvheadend;
+      isSystemUser = true;
+      group = "tvheadend";
     };
+    users.groups.tvheadend = {};
 
     systemd.services.tvheadend = {
       description = "Tvheadend TV streaming server";