Take nixpkgs.lib functions in poetryOverrides.nix from flake nixpkgs

It seems cleaner to me to just use the local flake input when we are
just pulling functions from lib. The consumer shouldn't need to provide it's
local packageset just for access to lib.

flake.nix

@@ -19,7 +19,7 @@
     ...
   } @ inputs: let
     no_system_outputs = {
-      poetryOverrides = import ./poetryOverrides.nix;
+      poetryOverrides = import ./poetryOverrides.nix { inherit nixpkgs; };
     };
 
     all_system_outputs = flake-utils.lib.eachDefaultSystem (system: let

poetryOverrides.nix

@@ -1,7 +1,7 @@
 # Copyright lowRISC Contributors.
 # Licensed under the MIT License, see LICENSE for details.
 # SPDX-License-Identifier: MIT
-{pkgs, ...}: let
+{nixpkgs, ...}: let
   # https://github.com/nix-community/poetry2nix/blob/master/docs/edgecases.md
   # poetry2nix tries to build the python packages based on the information
   # given in their own build description files (setup.py etc.)
@@ -61,7 +61,7 @@
     };
   };
 in
-  pkgs.lib.composeManyExtensions [
+  nixpkgs.lib.composeManyExtensions [
     preferwheel-overlay
     buildreqs-overlay
     dontwrap-overlay