Update head.hackage hash
This should fix the following error:
error: hash mismatch in fixed-output derivation '/nix/store/nzqbf3y4kqanwi2rq5h1n27qglhlmvmk-head.hackage.ghc.haskell.org.drv':
specified: sha256-7jhEysaHgczdNolOCuE9K6ZpLD3KJzsGSlgPHlZOvlg=
got: sha256-yFGpWo9ulwBEUyzTLKJBtbCWi7Ly4NEtocBRogdew9g=