nix experimental features

[agle]

Document error: experimental Lix feature 'flake-self-attrs' is disabled; use '--extra-experimental-features flake-self-attrs' to override

[katrinafyi]

This is troublesome because it's a Lix-specific flag. In Detsys Nix, this is enabled by default past a certain version.

It might be better to remove the self attr and ask users to use nix build .?submodule=true if they need the submodules? not sure.

[katrinafyi]

That said, if you see this error right now, you can add

extra-experimental-features = flake-self-attrs

to ~/.config/nix/nix.conf.