Update to Hackage revisions in Nix

A few days after I published Hackage revisions in Nix I got a comment from Wolfgang W that the next release of Nix will have a callHackageDirect with support for specifying revisions.

The code in PR #284490 makes callHackageDirect accept a rev argument. Like this:

haskellPackages.callHackageDirect {
  pkg = "openapi3";
  ver = "3.2.3";
  sha256 = "sha256-0F16o3oqOB5ri6KBdPFEFHB4dv1z+Pw6E5f1rwkqwi8=";
  rev = {
    revision = "4";
    sha256 = "sha256-a5C58iYrL7eAEHCzinICiJpbNTGwiOFFAYik28et7fI=";
  };
} { }

That's a lot better than using overrideCabal!