haskell-generic-builder: add an 'env' attribute to every Haskell expression...

haskell-generic-builder: add an 'env' attribute to every Haskell expression that can be used with nix-shell to create an interactive build environment for this package

For example:

  nix-shell --pure ~/.nix-defexpr -A haskell-ng.packages.ghc784.hsdns.env