Motivation for this change

@peti I just discovered that overriding haskellPackages by overriding haskell.packageOverrides was not working. This patch fixes that.

Things done

Tested using:

$ nix-build -E '(import ./. { overlays = [(final : previous : { haskell = previous.haskell // { packageOverrides = self : super : {blablabla = super.scientific;};};})];}).haskellPackages.blablabla'
/nix/store/s75xbfhn88187jk8238h4ii2ap9kg4m2-scientific-0.3.6.2

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.