debian11: Install crossbuild-essential-s390x

For testing s390x, ensuring that we have coverage of big-endian platforms.

images/debian.dhall

@@ -197,7 +197,7 @@ let debian11Images: List Image.Type =
     , extraPackages =
           ubuntuBuildDepends
         -- For cross-compilation testing
-        # [ "crossbuild-essential-arm64", "qemu-user" ]
+        # [ "crossbuild-essential-arm64", "crossbuild-essential-s390x", "qemu-user" ]
         # [ "libnuma-dev" ]
     }