default.nix

{ system ? builtins.currentSystem }:
let
  sources = import ./sources.nix;
  config = { allowUnfree = true; };
  overlays = [
    (self: super: {
      llvm = self.llvm.overrideAttrs (oldAttrs: {
        # introduce dirt to force rebuilds
        postInstall = oldAttrs.postInstall + "echo ${builtins.currentTime} > $out/dirt";
      });
    })
  ];
in
import sources.nixpkgs { inherit config overlays system; }