Haskell.nix can't build bootstrap aarch64-linux GHC 9.6.4 with unstable's stdenv.

Follow on from #2330, which was wrong in two critical ways (sorry about that...). Firstly, this is only a problem on aarch64-linux. Secondly, Nixpkgs can't do it either! Seems like GHC 9.6.4 doesn't build with GCC 14. 9.6.5 and 9.6.6 do though, so we could use one of those to bootstrap instead of 9.6.4, or we could patch 9.6.4, or we could use a different GCC.