haskell: generic-stack-builder sets an env var

generic-stack-builder was not setting STACK_PLATFORM_VARIANT="nix".
This is required by stack when handling its haskell packages database
snapshot. Stack must keep separated packages built with its own GHC
and those built with GHC from Nixpkgs.

pkgs/development/haskell-modules/generic-stack-builder.nix

@@ -16,6 +16,7 @@ stdenv.mkDerivation (args // {
     optional stdenv.isLinux glibcLocales ++
     [ ghc pkgconfig ];
 
+  STACK_PLATFORM_VARIANT="nix";
   STACK_IN_NIX_SHELL=1;
   STACK_IN_NIX_EXTRA_ARGS =
     concatMap (pkg: ["--extra-lib-dirs=${pkg}/lib"