stdenv bootstrap: fix evaluation

This is probably a fallout from #28557 merge and revert.
I can't see why exactly this happened, but it seems a safe fix.

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

@@ -126,9 +126,9 @@ rec {
 
         # Copy binutils.
         for i in as ld ar ranlib nm strip readelf objdump; do
-          cp ${binutils.binutils.out}/bin/$i $out/bin
+          cp ${binutils.out}/bin/$i $out/bin
         done
-        cp -d ${binutils.binutils.lib}/lib/lib*.so* $out/lib
+        cp -d ${binutils.lib}/lib/lib*.so* $out/lib
 
         chmod -R u+w $out