Don't require caller to know we have an overlay

nix/default.nix

-{ pkgs ? import <nixpkgs> { overlays = [ (import ./overlay.nix) ]; } }:
+{ pkgs ? import <nixpkgs> { } }:
 
 let
+  # Get our overlay in place regardless of whether a value is passed for pkgs.
+  # The build fails without it and it's unreasonable to expect our caller to
+  # know to apply it.
+  nixpkgs = import pkgs.path { overlays = [ (import ./overlay.nix) ]; };
+
   # Pin a particular version of haskell.nix.  The particular version isn't
   # special.  It's just recent at the time this expression was written and it
   # is known to work with PaymentServer.  It could be bumped if necessary but
   # this would probably only happen as a result of bumping the resolver in
   # stack.yaml.
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/0cb32e695d7014908fb01fd7e3d225ea33dbdc98.tar.gz) { inherit pkgs; };
+  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/0cb32e695d7014908fb01fd7e3d225ea33dbdc98.tar.gz) { pkgs = nixpkgs; };
 
   pkgSet = haskell.mkStackPkgSet {
     stack-pkgs = import ./pkgs.nix;