support flakes with extra experimental features flag

[phanirithvij]

Currently

nh os build -f '<nixpkgs/nixos>' -- --extra-experimental-features nix-command

Works.

Whereas

nh os build . -- --extra-experimental-features nix-command --extra-experimental-features flakes doesn't because there is a check for them to be enabled.

The workaround is to use NIX_CONFIG with experimental features set.

It would be nice to have it behave consistently so someone switching to flakes on a live ISO or newly installed system can have a good experience.