top-level: {build,host,target}Platform are defined in the stdenv instead

See #27069 for a discussion of this

pkgs/stdenv/nix/default.nix

@@ -10,10 +10,13 @@ bootStages ++ [
   (prevStage: let
     inherit (prevStage) stdenv;
   in {
-    inherit (prevStage) buildPlatform hostPlatform targetPlatform;
     inherit config overlays;
 
     stdenv = import ../generic rec {
+      buildPlatform = localSystem;
+      hostPlatform = localSystem;
+      targetPlatform = localSystem;
+
       inherit config;
 
       preHook = ''
@@ -30,8 +33,6 @@ bootStages ++ [
         nativeTools = false;
         nativePrefix = stdenv.lib.optionalString hostPlatform.isSunOS "/usr";
         nativeLibc = true;
-        hostPlatform = localSystem;
-        targetPlatform = localSystem;
         inherit stdenv;
         inherit (prevStage) binutils coreutils gnugrep;
         cc = prevStage.gcc.cc;