Add subsitituter config to .fleek.yml

[birdmanmandbir]

Is your feature request related to a problem? Please describe.
https://cache.nixos.org is extremely slow in my country, so I config subsitituters and trusted-users in my flake config, which could be fast

So I also want to config subsitituters in my fleek config

Describe the solution you'd like
Maybe just add subsitituters config and automatically add current user to trusted-users

Describe alternatives you've considered
use fleek eject

Additional context
A sample config:

  nixConfig = {
    experimental-features = [ "nix-command" "flakes" ];
    substituters = [
      # replace official cache with a mirror located in China
      "https://mirrors.ustc.edu.cn/nix-channels/store"
      "https://cache.nixos.org/"
    ];

    # nix community's cache server
    extra-substituters = [
      "https://nix-community.cachix.org"
    ];
    extra-trusted-public-keys = [
      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
    ];
  };

seems this config is different between nix and nix-darwin,
I'm using this in nix-darwin modules:

  nix.settings.substituters = [
    "https://mirrors.ustc.edu.cn/nix-channels/store"
  ];
  nix.settings.trusted-users = [
    "root"
    "username"
  ];