This one is passing nixpkgs-unstable
through the modules, on the other side you’re expecting pkgs-unstable
as an attribute:
Now you have basically two options: change the name of the attribute/parameter in both configuration and packages.nix or pass pkgs-unstable
in the specialArgs.
And the following is worth a read: Mixing stable and unstable packages on flake-based NixOS system - #4 by waffle8946
Does it help somehow?