nixos/elasticsearch: Set a group on the user

This fixed a problem I had when running ElasticSearch in an LXC
container, and it doesn't hurt using a dedicated group instead of
nogroup anyway.

nixos/modules/misc/ids.nix

···
       quassel = 89;
       amule = 90;
       minidlna = 91;
-      #elasticsearch = 92; # unused
+      elasticsearch = 92;
       #tcpcryptd = 93; # unused
       connman = 94;
       firebird = 95;

nixos/modules/services/search/elasticsearch.nix

···
 
     environment.systemPackages = [ cfg.package ];
 
-    users.extraUsers = singleton {
-      name = "elasticsearch";
-      uid = config.ids.uids.elasticsearch;
-      description = "Elasticsearch daemon user";
-      home = cfg.dataDir;
+    users = {
+      groups.elasticsearch.gid = config.ids.gids.elasticsearch;
+      users.elasticsearch = {
+        uid = config.ids.uids.elasticsearch;
+        description = "Elasticsearch daemon user";
+        home = cfg.dataDir;
+        group = "elasticsearch";
+      };
     };
   };
 }