Update haskell.nix to fix happy/hadrian build failures
Updates haskell.nix from 7e7550c (2025-12-09) to a0b48b5 (2026-02-05). This update includes: - hackage.nix update (b0bc4ec → 9f587a7) - hackage-for-stackage update (f3472c5 → 590439f) - iserv-proxy update (1506051 → 91ef7ff) - New HLS 2.12 support Expected to resolve build failures for: - happy-2.1.7 on aarch64-darwin - hadrian on aarch64-linux These were causing -minimal-ghc and -static variants to fail in CI.