[ET-VK] Only save_cache the first time (#7001)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6966

Add a check in `save_cache` to return early if the cache file already exists. Currently we append the same cache data to that file which makes no difference to model-load time.
ghstack-source-id: 254701486
@exported-using-ghexport

Differential Revision: [D66179919](https://our.internmc.facebook.com/intern/diff/D66179919/)

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