use nixpkgs-stackage in the build

instead of relying on it as an overlay

default.nix

-{ nixpkgs ? import <nixpkgs> {}
-}:
-with nixpkgs;
 let
+  stackageSrc = builtins.fetchTarball {
+    url = https://github.com/typeable/nixpkgs-stackage/archive/0c2597f1d300ea4d124c2078777d1bb598d6995f.tar.gz;
+    sha256 = "1imd6l6kc8ncw1jc7z9aj6spmm0a21l64ig45xfi0640df7v9x2r";
+  };
+
+  nixpkgs = import <nixpkgs> {
+    overlays = [ (import stackageSrc) ];
+  };
+
+  inherit (nixpkgs) pkgs lib;
+
   # This requires the nixpkgs-stackage overlay.
-  hpkgs = nixpkgs.pkgs.haskell.packages.stackage.lts-1110;
-  hlib = nixpkgs.pkgs.haskell.lib;
+  hpkgs = pkgs.haskell.packages.stackage.lts-1110;
+  hlib = pkgs.haskell.lib;
 
   # The gogol package is not part of stackage LTS, and it's a bit
   # tricky to build. Best results so far are with LTS specified above