ArtiNativeKeystoreConfig should derive its path from the state directory

Enabling the experimental-api feature and using TorClientConfigBuilder::from_directories fails on platforms without the XDG environment variables, because the added feature turns on the ArtiNativeKeystoreConfig, which has a path attribute, and the default of that attribute uses the XDG stuff even though explicit directories were provided.

To fix this, it should probably derive its path from the provided state directory, or from_directories() should set the keystore path explicitly.