`Derivation.used_entity()` returns the generated entity instead of the used one

`Derivation.used_entity()` in `cwlprov/prov.py` resolves the wrong identifier. The method reads `self.generated_entity_id` instead of `self.used_entity_id`, so it returns the same entity as `generated_entity()`.

https://github.com/common-workflow-language/cwlprov-py/blob/a318d73dd9/cwlprov/prov.py#L398-L400

```python
def used_entity(self) -> Union[Any, Entity, None]:
e_id = self.generated_entity_id
return e_id and self.provenance.entity(e_id)
```

Fix:
```python
def used_entity(self) -> Union[Any, Entity, None]:
e_id = self.used_entity_id
return e_id and self.provenance.entity(e_id)
```

Read cwlprov/prov.py around lines 398-400 and compare used_entity_id with generated_entity_id. Update the lookup so used_entity() resolves the used entity, then run the project's relevant tests to confirm it no longer returns the generated entity.

Written by the indexing model from the issue text.