GroupByKey iteration caching broken for portable runners like Dataflow runner v2

The wrong cache key is being used as it has not been namespaced to the state key.

This was previously being done within StateFetchingIterators but https://github.com/apache/beam/pull/17121 changed that to use a single shared key.

The fix is to subcache the cache before passing it into StateFetchingIterators restoring the prior behavior.