The issue was that the `AnimData.tmp_slot_handle` had no RNA property associated with it
and so it couldn't be saved in the lib override.
The fix is to add the property. To ensure the correct code is called, most of the previous code
was replaced with a call to `blender::animrig::assign_tmpaction`
Pull Request: https://projects.blender.org/blender/blender/pulls/141395
e80ceb52df