haskellPackages.cabal2nix-unstable: don't wrap with runtime deps

cabal2nix-unstable is mostly used for regenerating the Haskell package
set. Thus we should aim to make it quick to rebuild in case its hash
changes because of Haskell related changes.

- cabal2nix is not fussy about the Nix version it uses for nix-env(1)
  and we can just assume it is already in PATH like we do for
  maintainers/scripts/haskell/*.

- nix-prefetch-scripts causes the most trouble since its python
  dependencies depend on pandoc, so many Haskell changes require
  an additional Python rebuild when building cabal2nix-unstable.

  nix-prefetch-scripts is most likely installed and not necessary in
  many cases, e.g. hackage2nix doesn't need them which is the main use
  we have for cabal2nix-unstable. For the update scripts that do need
  them, we add them to the used nix-shell explicitly.

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