[ET-VK] Fix VulkanBackend `args` output index

Pull Request resolved: https://github.com/pytorch/executorch/pull/7297

The one in the `VK_CHECK_COND` expression was wrong. Use a variable for easier tracking.
ghstack-source-id: 257860131
@exported-using-ghexport

Differential Revision: [D67126361](https://our.internmc.facebook.com/intern/diff/D67126361/)

Co-authored-by: jorgep31415 <jorgepineda140@gmail.com>