Experimental features are now opt-in. There is currently one
experimental feature: "nix-command" (which enables the "nix"
command. This will allow us to merge experimental features more
quickly, without committing to supporting them indefinitely.
Typical usage:
$ nix build --experimental-features 'nix-command flakes' nixpkgs#hello
(cherry picked from commit 8e478c2341,
without the "flakes" feature)
aabf5c86c9