https://nixos.org/manual/nix/stable/#sec-conf-file
https://nixos.org/manual/nix/unstable/command-ref/conf-file.html

at nixos
/etc/nix/nix.conf -> is generated from nix.* options in /etc/nixos/configuration.nix

build-users-group = nixbld
trusted-users = root
allowed-users = *

system-features = nixos-test benchmark big-parallel kvm

experimental-features = nix-command flakes