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/darwin/make-bootstrap-tools.nix

@@ -332,10 +332,10 @@ in rec {
   };
 
   # The ultimate test: bootstrap a whole stdenv from the tools specified above and get a package set out of it
-  test-pkgs = let
-    stdenv = import (test-pkgspath + "/pkgs/stdenv/darwin") { inherit system bootstrapFiles; };
-  in import test-pkgspath {
+  test-pkgs = import test-pkgspath {
     inherit system;
-    bootStdenv = stdenv.stdenvDarwin;
+    stdenv = args: let
+        args' = args // { inherit bootstrapFiles; };
+      in (import (test-pkgspath + "/pkgs/stdenv/darwin") args').stdenvDarwin;
   };
 }

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 {

pkgs/top-level/default.nix

@@ -23,6 +23,10 @@
 , # Allow a configuration attribute set to be passed in as an argument.
   config ? {}
 
+, # 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
+
 , crossSystem ? null
 , platform ? assert false; null
 } @ args:
@@ -72,7 +76,7 @@ in let
     inherit lib nixpkgsFun;
   } // newArgs);
 
-  stdenv = import ../stdenv {
+  stdenv = (args.stdenv or (import ../stdenv)) {
     inherit lib allPackages system platform crossSystem config;
   };