freshBootstrapTools.build: put bootstrapFiles into stdenv-bootstrap-tools.nix

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

···
     inherit bootBinutils coreutilsMinimal tarMinimal busyboxMinimal bootGCC libc patchelf;
   };
 
-  bootstrapFiles = {
-    # Make them their own store paths to test that busybox still works when the binary is named /nix/store/HASH-busybox
-    busybox = runCommand "busybox" {} "cp ${build}/on-server/busybox $out";
-    bootstrapTools = runCommand "bootstrap-tools.tar.xz" {} "cp ${build}/on-server/bootstrap-tools.tar.xz $out";
-  };
+  inherit (build) bootstrapFiles;
 
   bootstrapTools =
     let extraAttrs = lib.optionalAttrs

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

···
   mpfr,
   patch,
   patchelf,
+  runCommand,
   tarMinimal,
   zlib,
 }:
···
   # ${libc.src}/sysdeps/unix/sysv/linux/loongarch/lp64/libnsl.abilist does not exist!
   withLibnsl = !stdenv.hostPlatform.isLoongArch64;
 in
-stdenv.mkDerivation {
+stdenv.mkDerivation (finalAttrs: {
   name = "stdenv-bootstrap-tools";
 
   meta = {
···
   # that we can safely copy them out of the store and to other
   # locations in the store.
   allowedReferences = [ ];
-}
+
+  passthru = {
+    bootstrapFiles = {
+      # Make them their own store paths to test that busybox still works when the binary is named /nix/store/HASH-busybox
+      busybox = runCommand "busybox" { } "cp ${finalAttrs.finalPackage}/on-server/busybox $out";
+      bootstrapTools =
+        runCommand "bootstrap-tools.tar.xz" { }
+          "cp ${finalAttrs.finalPackage}/on-server/bootstrap-tools.tar.xz $out";
+    };
+  };
+})