Update head.hackage hash (#248)
This should fix the following error:
error: hash mismatch in fixed-output derivation '/nix/store/4a81ynkkkxdg2zj29vf3nv9933n0ckxv-head.hackage.ghc.haskell.org.drv':
specified: sha256-yFGpWo9ulwBEUyzTLKJBtbCWi7Ly4NEtocBRogdew9g=
got: sha256-O6vUr/LlMiC9FtmGQg+1DiiTnhkru8Bkgtrjo+A06gw=
error: Cannot build '/nix/store/xa64sf0p9kxd57zbrn5jh4x6gnh7r22b-dot-cabal.drv'.