nixpkgs-bootstrap: get rid of "override"

this reduces the number of times we have to 'import' different nixpkgs during bootstrap

impure.nix

@@ -9,9 +9,11 @@
 }:
 let
   mkNixpkgs = import ./pkgs/by-name/nixpkgs-bootstrap/mkNixpkgs.nix {};
-  mkPkgs = branch: args: (
-    (mkNixpkgs (args // { inherit branch; })).pkgs
-  ).extend (import ./overlays/all.nix);
+  mkPkgs = branch: config: (
+    import ./pkgs/by-name/nixpkgs-bootstrap/${branch}.nix {
+      mkNixpkgs = args: mkNixpkgs (config // args);
+    }
+  ).pkgs.extend (import ./overlays/all.nix);
   pkgs = mkPkgs "master" { inherit localSystem; };
   inherit (pkgs) lib;
 

pkgs/by-name/nixpkgs-bootstrap/mkNixpkgs.nix

@@ -141,10 +141,6 @@ let
         inherit version;
         pname = "nixpkgs";
         passthru = (base.passthru or {}) // {
-          # override is used to configure hostPlatform higher up.
-          # TODO: refactor to avoid overriding.
-          override = overrideArgs: mkNixpkgs (args // overrideArgs);
-
           pkgs = nixpkgs;
 
           src = {