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/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;
   };