Accept per-project flake config

A project using devenv prompts: do you want to allow configuration setting 'extra-substituters' to be set to 'https://devenv.cachix.org' (y/N)?., but I cannot input y in the stdin. It happens in fish shell, and there is an issue: https://github.com/direnv/direnv/issues/1022.

The solution is to set accept-flake-config: true in the global nix setting (via):

{
  nix = {
    settings = {
      experimental-features = "nix-command flakes";
      accept-flake-config = true;
    };
  };
}