fix syntax issue

modules/nixos/user.nix

@@ -1,5 +1,8 @@
 { pkgs, username, ... }:
 
+let
+  binPath = "${pkgs.nix}/bin";
+in
 {
   nix.settings.trusted-users = [ username ];
 
@@ -23,8 +26,6 @@
     };
   };
 
-  binPath = "${pkgs.nix}/bin";
-
   # DO NOT promote the specified user to input password for `nix-store` and `nix-copy-closure`
   security.sudo = {
     # wheelNeedsPassword = false;
@@ -46,7 +47,7 @@
               options = [ "NOPASSWD" ];
             }
             {
-              command = "${binPath}/bin/poweroff";
+              command = "${binPath}/poweroff";
               options = [ "NOPASSWD" ];
             }
           ];