Change default cache path
# What does this PR do?

In Datasets, the default cache path ends up in `~/.cache/huggingface/datasets`, controlled by the environment variable `HF_HOME`. This PR uses the same env variable for the default cache path.
To avoid breaking changes:
- it still honors old environment variable names, if set
- if none is set, it moves the cache folder from the old location to the new one with a warning.

