generate-nix-overrides.hs: Use haskell.lib.compose

This is passed in as haskellLib, as in convention in nixpkgs, to allow easy of
calling the generated override file from
pkgs/development/haskell-modules/default.nix in nixpkgs.

haskell.lib.compose is a set of "more composable" haskell library functions,
they take the "pkg" argument last.

See https://github.com/NixOS/nixpkgs/pull/142940 for more info.