Fix the environment variable for huggingface cache
Hi ! yes now `datasets` uses `huggingface_hub` to download and cache files from the HF Hub so you need to use `HF_HOME` (or manually `HF_HUB_CACHE` and `HF_DATASETS_CACHE` if you want to separate HF Hub cached files and cached datasets Arrow files)

So in your change I guess it needs to be `HF_HOME` instead of `HF_CACHE` ? Thank you for your comment. You are right. I am sorry for my mistake, I fixed it. The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7200). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. I just had this issue, and needed to move the setting the env code in the python file to top, before the import of the lib 
ie. 
```python
import os
LOCAL_DISK_MOUNT = '/mnt/data'

os.environ['HF_HOME'] = f'{LOCAL_DISK_MOUNT}/hf_cache/'
os.environ['HF_DATASETS_CACHE'] = f'{LOCAL_DISK_MOUNT}/datasets/'

from datasets import load_dataset
from datasets import load_dataset_builder
from psutil._common import bytes2human


```
Resolve  #6256. As far as I tested, `HF_DATASETS_CACHE` was ignored and I could not specify the cache directory at all except for the default one by this environment variable. `HF_HOME` has worked. Perhaps the recent change on file downloading by `huggingface_hub` could affect this bug.

In my testing, I could not specify the cache directory even by `load_dataset("dataset_name" cache_dir="...")`. It might be another issue. I also welcome any advice to solve this issue.