I quite like flakes on the whole, but there is still, in my opinion, a pretty big unsolved issue, in that it’s impossible to configure a flake. Some consider it a feature, and in some sense it is, but at the same time it incurs some pretty hefty drawbacks (admittedly, these are much more relevant for using nix as a build tool rather than as a package manager for an OS).

The packages you specify via the packages attribute are the ones it will build dependencies for, but not build. You want to return a list of packages selected from the argument attribute set, like

packages = hsPkgs: [hsPkgs.pkgA, hsPkgs.pkgB];