add graphical environment as requirement

the instructions say that the command will open a window, which
naturally requires a graphical environment. but this is very late in the
process, and will likely not be obvious in the middle of a a self-directed learning situation.

source/tutorials/nixos/nixos-configuration-on-vm.md

@@ -17,7 +17,7 @@ Virtual machines are a practical tool for debugging NixOS configurations.
 
 ## What do you need?
 
-- A working [Nix installation](https://nixos.org/manual/nix/stable/installation/installation.html) on Linux, or [NixOS](https://nixos.org/manual/nixos/stable/index.html#sec-installation)
+- A working [Nix installation](https://nixos.org/manual/nix/stable/installation/installation.html) on Linux, or [NixOS](https://nixos.org/manual/nixos/stable/index.html#sec-installation), with a graphical environment
 - Basic knowledge of the [Nix language](reading-nix-language)
 
 ## Starting from the default NixOS configuration