The command...
nix-build -A config.system.build.isoImage -I nixos-config=installer.nix nixos
...creates an .iso file in the ./result directory. You can then copy this onto a
USB and use it a custom installer...
cp ./result/iso/*-linux.iso /dev/sda
I needed an installer that used a version of the Linux kernel higher than the
one distributed on NixOS's website: 4.19.? -> 5.4.20+. My Acer laptop needed a
version of the kernel that supported its network controller: Intel 3168NGW.
TODO(wpcarro): Pin the nixpkgs git commit SHA inside of installer.nix.
