rustc: add note about libiconv dependency

Author: winterqt

pkgs/development/compilers/rust/rustc.nix

@@ -163,6 +163,7 @@ in stdenv.mkDerivation rec {
   ];
 
   buildInputs = [ openssl ]
+    # TODO: remove libiconv once 1.66 is used to bootstrap
     ++ optionals stdenv.isDarwin [ libiconv Security ]
     ++ optional (!withBundledLLVM) llvmShared;