Fix cabal-install of ghc package

!10547 (closed) has regressed support for building ghc library using cabal-install as part of the build now needs to be generated using a python script (which is not called from Setup.hs).

Either we need to revert this patch or fix the Setup.hs.

I am not sure if we should add a build dependency on python like this though. An alternative is to rewrite the generation script in Haskell so we can depend on it using build-tool-depends like the normal primops generation logic.