top-level: Allow manually specifying a stdenv, and fix stdenv tests

- The darwin test can now force the use of the freshly-booted darwin stdenv
 - The linux test now passes enough dummy arguments

This may make debugging harder, if so, check out #20889

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

@@ -175,6 +175,13 @@ rec {
 
   bootstrapTools = (import ./default.nix {
     inherit system bootstrapFiles;
+
+    lib = assert false; null;
+    allPackages = assert false; null;
+
+    platform = assert false; null;
+    crossSystem = assert false; null;
+    config = assert false; null;
   }).bootstrapTools;
 
   test = derivation {