shen-sbcl: mark as broken

- the build's been broken for about a year
  https://hydra.nixos.org/job/nixpkgs/trunk/shen-sbcl.x86_64-linux/all
- noone minds the failure apparently (trying to fix)
- it still seems relatively expensive to repeat these failures

pkgs/development/interpreters/shen-sbcl/default.nix

@@ -36,6 +36,7 @@ stdenv.mkDerivation rec {
     changelog = "https://github.com/Shen-Language/shen-cl/raw/v${version}/CHANGELOG.md";
     platforms = sbcl.meta.platforms;
     maintainers = with maintainers; [ bsima ];
+    broken = true;
     license = licenses.bsd3;
   };
 }