callHackage: make 'all-cabal-hashes' repository overridable

This commit changes callHackage to use a deterministic version of the Hackage
checkout from https://github.com/commercialhaskell/all-cabal-hashes by default.
This means that packages uploaded to Hackage after today will be available to
callHackage only after "pkgs/data/misc/hackage/default.nix" has been updated.

People who want the previous behavior where we always had the latest version of
Hackage available -- at the cost of frequent downloads from Github --, can add
the following override to their "~/.nixpkgs/config.nix" file:

  {
    packageOverrides = super: {
      all-cabal-hashes = builtins.fetchTarball "https://github.com/commercialhaskell/all-cabal-hashes/archive/hackage.tar.gz";
    };
  }