mono: Disable on aarch64

mono40: https://hydra.nixos.org/build/67660031
mono44: https://hydra.nixos.org/build/67666036
mono50: https://hydra.nixos.org/build/67825397
mono54: https://hydra.nixos.org/build/67825452

pkgs/development/compilers/mono/generic.nix

···
   meta = {
     homepage = http://mono-project.com/;
     description = "Cross platform, open source .NET development framework";
-    platforms = with stdenv.lib.platforms; darwin ++ linux;
+    platforms = with stdenv.lib.platforms; allBut [ "aarch64-linux" ];
     maintainers = with stdenv.lib.maintainers; [ viric thoughtpolice obadz vrthra ];
     license = stdenv.lib.licenses.free; # Combination of LGPL/X11/GPL ?
   };