Implementation of `dataset_builder_safe` is not thread safe

In the `discovery.py` module, the [dataset_builder_safe](https://github.com/huggingface/faceberg/blob/2346ee32ba68a7f906e1244d4b9056c404f35722/faceberg/discover.py#L18) uses `os.chdir()` to change the current working directory for the entire Python process. Other functions such as `write_manifest()` apply `ThreadPoolExecutor` for parallel I/O work. If two threads are working at the same time, one thread can change the working directory while another thread is trying to access a relative path. This leads to `FileNotFoundError` errors which are hard to diagnose. WDYT?

Start by reading the dataset_builder_safe implementation in the referenced discovery.py/discover.py module, then inspect write_manifest and its ThreadPoolExecutor use. Trace how os.chdir() and relative paths interact during concurrent I/O; done means parallel work no longer encounters working-directory-dependent FileNotFoundError failures.

Written by the indexing model from the issue text.