Update haskell.nix to fix head.hackage FOD hash mismatch
The head.hackage upstream content changed after Feb 21, making the fixed-output derivation hash stale. With the cached outputs GC'd, all JS backend IFD evaluations fail with hash mismatch, blocking the entire Hydra eval (no jobsetevals entry produced). Update haskell.nix from 7e7550c (Dec 9 2025) to cc939d0 (Feb 23 2026) which includes the daily "Update Hackage and Stackage" with the current head.hackage hash.