Update message to reflect nix 2.0 availability of imperative nix-update (#36123)

default.nix

@@ -6,7 +6,10 @@ if ! builtins ? nixVersion || builtins.compareVersions requiredVersion builtins.
 
     This version of Nixpkgs requires Nix >= ${requiredVersion}, please upgrade:
 
-    - If you are running NixOS, use `nixos-rebuild' to upgrade your system.
+    - If you are running NixOS, `nixos-rebuild' can be used to upgrade your system.
+
+    - Alternatively, with Nix > 2.0 `nix upgrade-nix' can be used to imperatively
+      upgrade Nix. You may use `nix-env --version' to check which version you have.
 
     - If you installed Nix using the install script (https://nixos.org/nix/install),
       it is safe to upgrade by running it again: