top-level: avoid another assert false while we're at it

pkgs/stdenv/darwin/make-bootstrap-tools.nix

@@ -334,7 +334,7 @@ in rec {
   # The ultimate test: bootstrap a whole stdenv from the tools specified above and get a package set out of it
   test-pkgs = import test-pkgspath {
     inherit system;
-    stdenv = args: let
+    stdenvFunc = args: let
         args' = args // { inherit bootstrapFiles; };
       in (import (test-pkgspath + "/pkgs/stdenv/darwin") args').stdenvDarwin;
   };

pkgs/top-level/default.nix

@@ -25,7 +25,7 @@
 
 , # The standard environment for building packages, or rather a function
   # providing it. See below for the arguments given to that function.
-  stdenv ? assert false; null
+  stdenvFunc ? import ../stdenv
 
 , crossSystem ? null
 , platform ? assert false; null
@@ -76,7 +76,7 @@ in let
     inherit lib nixpkgsFun;
   } // newArgs);
 
-  stdenv = (args.stdenv or (import ../stdenv)) {
+  stdenv = stdenvFunc {
     inherit lib allPackages system platform crossSystem config;
   };