feat: change desktop's default shell to nushell

modules/nixos/core-desktop.nix

@@ -241,6 +241,14 @@
     ];
   };
 
+  # add user's shell into /etc/shells
+  environment.shells = with pkgs; [
+    bash
+    nushell
+  ];
+  # set user's default shell system-wide
+  users.defaultUserShell = pkgs.nushell;
+
   # for power management
   services.upower.enable = true;
 }