nixos/clamav: set "clamav" user's primary group to "clamav"

So that the files created by the clamav service is owned by group
"clamav" instead of "nogroup".

nixos/modules/services/security/clamav.nix

@@ -81,6 +81,7 @@ in
     users.extraUsers = singleton {
       name = clamavUser;
       uid = config.ids.uids.clamav;
+      group = clamavGroup;
       description = "ClamAV daemon user";
       home = stateDir;
     };