beamPackages: use callPackagesWith from nixpkgs lib
beamPackages: use callPackagesWith from nixpkgs lib

2f78ee7e816ae39f86bae2e6c61b2d14d3fe3ff4 accidentally changed this from
(nixpkgs) stdenv.lib to (beam-modules) lib', which broke things.
This changes it back to (nixpkgs) lib.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs