cabal2nix: fix build

Without this change, the build fails with:

```
Warning:
    This package indirectly depends on multiple versions of the same package. This is very likely to cause a compile failure.
      package hackage-db (hackage-db-2.0.1-AliTRXEJBGFAEMpONOEnbV) requires Cabal-2.4.0.1
      package hpack (hpack-0.32.0-1iJuukTWmC4EGkYhAiFeII) requires Cabal-3.0.0.0-CD1INMNwoxXIKVgcu1nec5
      package distribution-nixpkgs (distribution-nixpkgs-1.3.1-82Z2DCdLAUs8Q2gjtlPLz5) requires Cabal-3.0.0.0-CD1INMNwoxXIKVgcu1nec5
      package cabal2nix (cabal2nix-2.15.0) requires Cabal-3.0.0.0-CD1INMNwoxXIKVgcu1nec5
      package cabal2nix (cabal2nix-2.15.0) requires Cabal-3.0.0.0-CD1INMNwoxXIKVgcu1nec5
      package cabal2nix (cabal2nix-2.15.0) requires Cabal-3.0.0.0-CD1INMNwoxXIKVgcu1nec5
```