prometheus/exporters: fix regression in DynamicUser behavior

Instead of setting User/Group only when DynamicUser is disabled, the
previous version of the code set it only when it was enabled. This
caused services with DynamicUser enabled to actually run as nobody, and
services without DynamicUser enabled to run as root.

Regression from fbb7e0c.

nixos/modules/services/monitoring/prometheus/exporters.nix

@@ -127,7 +127,7 @@ let
         serviceConfig.Restart = mkDefault "always";
         serviceConfig.PrivateTmp = mkDefault true;
         serviceConfig.WorkingDirectory = mkDefault /tmp;
-      } serviceOpts ] ++ optional (serviceOpts.serviceConfig.DynamicUser or false) {
+      } serviceOpts ] ++ optional (!(serviceOpts.serviceConfig.DynamicUser or false)) {
         serviceConfig.User = conf.user;
         serviceConfig.Group = conf.group;
       });