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

See #27069 for a discussion of this

pkgs/stdenv/freebsd/default.nix

@@ -35,6 +35,9 @@ let inherit (localSystem) system; in
 
     stdenv = import ../generic {
       name = "stdenv-freebsd-boot-1";
+      buildPlatform = localSystem;
+      hostPlatform = localSystem;
+      targetPlatform = localSystem;
       inherit config;
       initialPath = [ "/" "/usr" ];
       hostPlatform = localSystem;
@@ -52,6 +55,9 @@ let inherit (localSystem) system; in
 
     stdenv = import ../generic {
       name = "stdenv-freebsd-boot-0";
+      buildPlatform = localSystem;
+      hostPlatform = localSystem;
+      targetPlatform = localSystem;
       inherit config;
       initialPath = [ prevStage.bootstrapTools ];
       inherit (prevStage.stdenv)
@@ -62,12 +68,12 @@ let inherit (localSystem) system; in
   })
 
   (prevStage: {
-    buildPlatform = localSystem;
-    hostPlatform = localSystem;
-    targetPlatform = localSystem;
     inherit config overlays;
     stdenv = import ../generic {
       name = "stdenv-freebsd-boot-3";
+      buildPlatform = localSystem;
+      hostPlatform = localSystem;
+      targetPlatform = localSystem;
       inherit config;
 
       inherit (prevStage.stdenv)
@@ -77,8 +83,6 @@ let inherit (localSystem) system; in
         nativeTools  = true;
         nativePrefix = "/usr";
         nativeLibc   = true;
-        hostPlatform = localSystem;
-        targetPlatform = localSystem;
         inherit (prevStage) stdenv;
         cc           = {
           name    = "clang-9.9.9";