hydra-module: honor user and group ids

nixos/modules/services/continuous-integration/hydra/default.nix

···
 
   config = mkIf cfg.enable {
 
-    users.extraGroups.hydra = { };
+    users.extraGroups.hydra = {
+      gid = config.ids.gids.hydra;
+    };
 
     users.extraUsers.hydra =
       { description = "Hydra";
···
         createHome = true;
         home = baseDir;
         useDefaultShell = true;
+        uid = config.ids.uids.hydra;
       };
 
     users.extraUsers.hydra-queue-runner =
···
         group = "hydra";
         useDefaultShell = true;
         home = "${baseDir}/queue-runner"; # really only to keep SSH happy
+        uid = config.ids.uids.hydra-queue-runner;
       };
 
     users.extraUsers.hydra-www =
       { description = "Hydra web server";
         group = "hydra";
         useDefaultShell = true;
+        uid = config.ids.uids.hydra-www;
       };
 
     nix.trustedUsers = [ "hydra-queue-runner" ];