yarn2nix: Don't pass yarnNix to mkDerivation (#92856)

The generated yarnNix file doesn't need to be part of the mkDerivation.
And doing so prevents other platforms from reproducibly instantiating
it. With this change you can e.g. do

  darwinPkgs.yarn2nix.mkYarnPackage {
    # ...
    yarnNix = pkgs.yarn2nix.mkYarnNix {
      yarnLock = ./yarn.lock;
    };
  }

Which is a darwin derivation, but can still be instantiated reproducibly on Linux.