nixos: use usercfg.home.username for username

Use `usercfg.home.username` for username instead of attribute name,
as this way we can change username regardless of the name of the attribute.

nixos/default.nix

@@ -60,7 +60,10 @@ in
       }) cfg.users
     );
 
-    systemd.services = mapAttrs' (username: usercfg:
+    systemd.services = mapAttrs' (_: usercfg:
+      let
+        username = usercfg.home.username;
+      in
         nameValuePair ("home-manager-${utils.escapeSystemdPath username}") {
           description = "Home Manager environment for ${username}";
           wantedBy = [ "multi-user.target" ];