Bootstrap with ghc966

[TravisWhitaker]

No description provided.

[TravisWhitaker]

Meant to fix #2337

[TravisWhitaker]

Please double-check my work; I'm not terribly familiar with the materialization machinery and cargo-culted this based on what error messages told me.

[hamishmack]

We probably still want to use 9.6.4 when 9.6.6 is not present in nixpkgs. Something like:

       compiler-nix-name =
         if builtins.compareVersions ghc-version "9.4.7" < 0
           then "ghc928"
+        else if buildPackages.haskell.compiler ? ghc966
+          then "ghc966"
         else if buildPackages.haskell.compiler ? ghc964
           then "ghc964"
         else "ghc962";

I spent some time trying to update the materializations and it's a bit of a nightmare to get right. In particular the plans for 9.8.2 and 9.8.4 versions of hadrian (built with 9.6.6) are different (where they were the same for 9.6.4). I think we should just drop the plans for now, see #2339. We can put them back if it turns out to cause significant evaluation time slow downs.

[hamishmack]

Now that #2339 is merged I have opened #2342 to replace this PR.