Tracking issue for full GHC 9.8 support

[mmhat]

In #2637 a preliminary support for GHC 9.8 was added to the project (See the stack.ghc-9.8.yaml file).
However, dhall-nix and dhall-nixpkgs were excluded since the hnix dependency of these packages does not build with GHC 9.8.
This issue serves as a tracking issue to implement GHC 9.8 support for those packages as well.
Once the hnix development caught up we need to

• add dhall-nix to the packages section of the stack.ghc-9.8.yaml file.
• remove the conditional in the cabal.project file.
• adjust our GitHub Workflow such that it builds dhall-nix with GHC 9.8 as well.