Nix feature should enable "experimental-features = nix-command flakes" by default

[vorburger]

While I understand that Nix flakes are technically still "experimental", IMHO at this point it makes sense to enable them by default.

As per e.g. https://github.com/vorburger/nixfiles/blob/develop/docs/docs/reference/nix-cli.md (and similarly much elsewhere around the Internets), IMHO doing something like this during setup would ease the initial user experience:

echo "experimental-features = nix-command flakes" >> ~/.config/nix/nix.conf

[vorburger]

Ah right, RTFM! 😆 https://github.com/devcontainers/features/tree/main/src/nix#options

"ghcr.io/devcontainers/features/nix:1": {
  "extraNixConfig": "experimental-features = nix-command flakes"
},

Does the trick... but IMHO it would be neat (at this stage of Nix flakes) to make this the default.

@Chuxel