Fix: PluginVersionProvider should cache process class (#4683)

Currently, the `PluginVersionProvider` is caching process instance, rather than class.
This commit fixes the bug, meaning the cache will now work correctly.
Removing the reference to the process instance also is a step towards allowing it to be garbage collected.

aiida/engine/processes/process.py

@@ -672,7 +672,7 @@ def _setup_db_record(self) -> None:
 
     def _setup_metadata(self) -> None:
         """Store the metadata on the ProcessNode."""
-        version_info = self.runner.plugin_version_provider.get_version_info(self)
+        version_info = self.runner.plugin_version_provider.get_version_info(self.__class__)
         self.node.set_attribute_many(version_info)
 
         for name, metadata in self.metadata.items():