[flake] downgrade haskell.nix according to julm's remarks

nix flake lock --override-input haskell-nix github:input-output-hk/haskell.nix/a921f7340c0c1a561f0e743d4b07b8130ea5c5be
parent 284ea992
Pipeline #6837 failed with stages
in 15 minutes and 18 seconds