ghc-8.4.1: un-revert "update to 8.4.1-alpha2"
This commit undoes the revert from a74b0e717d9156c61264e15a1e7fed680a5e4b31.
See https://github.com/NixOS/nixpkgs/issues/34229 and
https://github.com/NixOS/nixpkgs/pull/34232 for further details about why this
build seemed broken.