Provide HASKELL_PACKAGES override for all nix-eval runners

[Chris Warburton]

We run QuickSpec using nix-eval, which provides NIX_EVAL_HASKELL_PKGS as
an override if we want to alter the source of Haskell packages. Since
our QuickSpec runners hard-code NIX_EVAL_HASKELL_PKGS for our own
purposes, this prevents users specifying their own override.

In the case of nix-support/quickspecEnv.nix, we provide our own override
HASKELL_PACKAGES. This is good, and should be made available by our
other hard-coded values of NIX_EVAL_HASKELL_PKGS too.