asymptote: mark broken on darwin

It only builds with gc-7.6.0 on darwin for some reason.

/cc ZHF #45961

pkgs/tools/graphics/asymptote/default.nix

@@ -63,6 +63,7 @@ stdenv.mkDerivation {
     description =  "A tool for programming graphics intended to replace Metapost";
     license = licenses.gpl3Plus;
     maintainers = [ maintainers.raskin maintainers.peti ];
+    broken = stdenv.isDarwin;  # https://github.com/vectorgraphics/asymptote/issues/69
     platforms = platforms.linux ++ platforms.darwin;
   };
 }