Instead of relying on the separate cache implementations to check for the
existence of cached products, we now explicitly track whether or not the execution
is the first time the action has been invoked for a job.
This is a pure refactor, moving from a separate .cache file per bundle to a single cache-metadata.json file describing all bundles. Instead of storing cache metadata in a separate .cache file per artifact bundle, all of the metadata is now stored in a single `.json` file.
This will make it easier to implement more flexible artifact-caching strategies, such as caching each wrapper zip separately.
* Always include cache protocol version in cache key
* Store all cache metadata in a single JSON file
* Rename cache-metadata file and bump protocol version
* Polish and documentation
This will eliminate cache entries from previous workflow runs, allowing
us to test cache functionality in isolation. If the `CACHE_KEY_SEED` environment
variable is not set, this will have no impact.
