flake reproducing npmlock2nix cache issue

flake.nix

{
  inputs = rec {
    nixpkgs.url = "nixpkgs";
    flake-utils.url = "github:numtide/flake-utils";
    sabakiSrc.url = "github:SabakiHQ/Sabaki";
    sabakiSrc.flake = false;
    npmlock2nix.url = "github:winston0410/npmlock2nix/issue113";
    npmlock2nix.flake = false;
  };
  outputs = { self, nixpkgs, flake-utils
            , npmlock2nix
            , sabakiSrc, ... }:
    let
      supportedSystems = [ "x86_64-linux" ];
    in
      flake-utils.lib.eachSystem supportedSystems (system:
      let
        pkgs = import nixpkgs { inherit system; };

        sabaki = (pkgs.callPackage npmlock2nix { }).node_modules {
          src = sabakiSrc;
          # see https://discourse.nixos.org/t/cant-setup-development-with-electron-error-spawn-enoent/18497/6
          buildCommands = [ ''
            if [ -n "$http_proxy" ]; then
              export ELECTRON_GET_USE_PROXY=1
              export GLOBAL_AGENT_HTTP_PROXY=''${http_proxy}
              export GLOBAL_AGENT_HTTPS_PROXY=''${http_proxy}
            fi
            # build node-red
            npm install 
            npm run build
            '' ];
        };
      in {
        packages.default = sabaki;
      });
}