doc: clarify building-nixos CD/DVD instructions

This change updates the instructions for building a NixOS ISO so that it's clear how to do it.

Previously, the instructions stated to set NIXOS_CONFIG prior to running `nix-build`, yet the example provided by-passed NIXOS_CONFIG anyway. But the *really* important missing piece is the need for nixos/default.nix. See #21840.

This change removes the NIXOS_CONFIG verbiage, and adds steps to clone nixpkgs and (most importantly) cd'ing into nixpkgs/nixos. That way, the reader may think: *Oh, so I need a default.nix and a configuration.nix. Ahhh, OK.*

I purposely added the redundant default.nix argument.