Pin nixpkgs in flake.nix

This is to work around an issue where hydra was trying to update the flake.lock to a newer version of nixpkgs.

flake.nix

 {
   description = "head.hackage";
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/33e0d99cbedf2acfd7340d2150837fbb28039a64";
   inputs.ghc-artefact-nix.url = "github:mpickering/ghc-artefact-nix";
   inputs.ghc-artefact-nix.flake = false;
   inputs.all-cabal-hashes.url = "github:commercialhaskell/all-cabal-hashes";