Use nix-index from a fixed output derivation does not work

[vkryachko]

Hi, not sure if it's a known issue, but it would be great if output of nix-index on the same revision of nixpkgs would produce the same hash, it is not the case today, i.e. try running to this flake:

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

  outputs = { self, nixpkgs}: 
    let pkgs = nixpkgs.legacyPackages.x86_64-linux;
    in {
      packages.x86_64-linux.default = pkgs.runCommand "my-index" {
        nativeBuildInputs = [ pkgs.nix-index pkgs.nix ];
        outputHash = "";
        outputHashAlgo = "sha256";
        outputHashMode = "recursive";
      } ''
        export NIX_STATE_DIR="$(mktemp -d)"
        nix-shell || true # makes nix populate state dir

        nix-index --db $out -s x86_64-linux --filter-prefix '/bin/' -f ${nixpkgs}

      '';
    };
}

Produced hash will be different every time