factor out pkgsFun as an optional argument

src/default.nix

@@ -1,13 +1,15 @@
+{ pkgsFun ? import <nixpkgs>
+}:
 let
   lib = import <nixpkgs/lib>;
   nixpkgsArgs = { config.allowNonSource = false; };
-  nixpkgsOnBuildForBuild  = import <nixpkgs> nixpkgsArgs;
+  nixpkgsOnBuildForBuild  = pkgsFun nixpkgsArgs;
 
   ownerboot = { hostPlatform?null, overlay }:
     (lib.makeScope lib.callPackageWith (self: {
       inherit lib nixpkgsOnBuildForBuild;
 
-      nixpkgsOnBuildForHost   = import <nixpkgs>
+      nixpkgsOnBuildForHost   = pkgsFun
         (nixpkgsArgs // (lib.optionalAttrs (hostPlatform!=null) {
           crossSystem = hostPlatform; }));