[HfFileSystem] minor fix

[lhoestq]

this could happen when listing a parent directory of an already explored directory:

  File "/Users/quentinlhoest/.pyenv/versions/hf-datasets/lib/python3.12/site-packages/fsspec/spec.py", line 637, in glob
    allpaths = self.find(root, maxdepth=depth, withdirs=True, detail=True, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/quentinlhoest/hf/huggingface_hub/src/huggingface_hub/hf_file_system.py", line 567, in find
    out = self._ls_tree(
          ^^^^^^^^^^^^^^
  File "/Users/quentinlhoest/hf/huggingface_hub/src/huggingface_hub/hf_file_system.py", line 448, in _ls_tree
RuntimeError: dictionary changed size during iteration

[HuggingFaceDocBuilderDev]

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