Update nix flake to support Cabal 3.12 for deployment

stackage-server needs to be updated to support Cabal 3.12 syntax. Currently, there is an open PR that updates the stack.yaml to use a newer version. However, for deployment, the nix flake must also be updated, as it is used in production deployments.

(Following is LLM generated):

Action items:
- Update the nix flake to pull in a newer version of cabal (3.12), using an appropriate override.
- Ensure the deployment process works using the updated nix flake and cabal version.
- Reference and coordinate with the open PR that bumps stack.yaml to ensure consistency between local development (stack) and deployment (nix).

This is important for continued compatibility with new Cabal syntax and tooling.