haskell.compiler.ghcHEAD: bootstrap using 8.10.7 binary

This should shorten the bootstrapping part (not requiring an
intermediate 9.0.1 build). Seems like 8.10.7 is still supported for
bootstrapping for the moment and allows compiling for aarch64-darwin.

pkgs/top-level/haskell-packages.nix

···
       llvmPackages = pkgs.llvmPackages_10;
     };
     ghcHEAD = callPackage ../development/compilers/ghc/head.nix {
-      bootPkgs = packages.ghc901; # no binary yet
+      bootPkgs = packages.ghc8107Binary;
       inherit (buildPackages.python3Packages) sphinx;
       # Need to use apple's patched xattr until
       # https://github.com/xattr/xattr/issues/44 and