Add cabal update to horizon-gen-nix CI job

Summary

• Add cabal update before horizon-gen-nix in CI to ensure fresh package index
• New horizon-gen-nix.cabal option (types.package, default: nixpkgs#cabal-install)
• Follows same pattern as omnix/cachix modules for package options

Fixes stale cabal index causing horizon-gen-nix to fail resolving newly added packages on CI runners.

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