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.
* manual: Mark commands that require root
Mark every command that requires to be run as root by prefixing them
with '#' instead of '$'.
* manual: Add note about commands that require root
