go-sct: disable on Darwin

It does not support Darwin.

pkgs/tools/X11/go-sct/default.nix

···
     description = "Color temperature setting library and CLI that operates in a similar way to f.lux and Redshift";
     license = licenses.mit;
     maintainers = with maintainers; [ cstrahan ];
-    platforms = platforms.unix;
+    platforms = platforms.linux;
   };
 }