nixos/modules/config/resolvconf.nix: skip systemPackages if disabled

A change made in #166308 added `networking.resolvconf.package` to the
`environment.systemPackages` list, so it is installed as part of the
system image. However it does so unconditionally, meaning that even if
the `config.networking.resolvconf.enable` is set to false the package
listed in the `networking.resolvconf.package` would still be intalled.

This change makes it so the package installation will depend on the
status of the `config.networking.resolvconf.enable` option instead.

nixos/modules/config/resolvconf.nix

@@ -132,13 +132,13 @@ in
             exit 1
           ''
         else configText;
-
-      environment.systemPackages = [ cfg.package ];
     }
 
     (mkIf cfg.enable {
       networking.resolvconf.package = pkgs.openresolv;
 
+      environment.systemPackages = [ cfg.package ];
+
       systemd.services.resolvconf = {
         description = "resolvconf update";