Avoid possible infinite recursion

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

hosts/common/user/configs/gui/networking/default.nix

@@ -1,9 +1,9 @@
 {
-  user ? throw "user argument is required",
+  username ? throw "username argument is required",
 }:
 { ... }:
 {
-  home-manager.users.${user.name} = {
+  home-manager.users.${username} = {
     services.network-manager-applet.enable = true;
     systemd.user.services.network-manager-applet.Unit.After = [ "graphical-session.target" ];
   };