`Permission denied` when sharing cache between users

### Describe the bug

We want to use `datasets` and `transformers` on a shared machine. Right now, each user has a separate HF_HOME in their home directory. To reduce duplicates of the datasets, we want to share that cache. While experimenting, we are running into `Permission denied` errors.

It looks like this was supported in the past (see #6589)?

Is there a correct way to share caches across users?

### Steps to reproduce the bug

1. Create a directory `/models/hf_hub_shared_experiment` with read/write permissions for two different users
2. For each user run the script below

```python
import os

os.environ["HF_HOME"] = "/models/hf_hub_shared_experiment"
os.environ["HF_DATASETS_CACHE"] = "/models/hf_hub_shared_experiment/data"

import datasets
import transformers

DATASET = "tatsu-lab/alpaca"
MODEL = "meta-llama/Llama-3.2-1B-Instruct"

model = transformers.AutoModelForCausalLM.from_pretrained(MODEL)
tokenizer = transformers.AutoTokenizer.from_pretrained(MODEL)
dataset = datasets.load_dataset(DATASET)
```

The first user is able to download and use the model and dataset. The second user gets these errors:

```
$ python ./experiment_with_shared.py
Could not cache non-existence of file. Will ignore error and continue. Error: [Errno 13] Permission denied: '/models/hf_hub_shared_experiment/hub/models--meta-llama--Llama-3.2-1B-Instruct/.no_exist/9213176726f574b556790deb65791e0c5aa438b6/custom_generate/generate.py'
Could not cache non-existence of file. Will ignore error and continue. Error: [Errno 13] Permission denied: '/models/hf_hub_shared_experiment/hub/datasets--tatsu-lab--alpaca/.no_exist/dce01c9b08f87459cf36a430d809084718273017/alpaca.py'
Could not cache non-existence of file. Will ignore error and continue. Error: [Errno 13] Permission denied: '/models/hf_hub_shared_experiment/hub/datasets--tatsu-lab--alpaca/.no_exist/dce01c9b08f87459cf36a430d809084718273017/.huggingface.yaml'
Could not cache non-existence of file. Will ignore error and continue. Error: [Errno 13] Permission denied: '/models/hf_hub_shared_experiment/hub/datasets--tatsu-lab--alpaca/.no_exist/dce01c9b08f87459cf36a430d809084718273017/dataset_infos.json'
Traceback (most recent call last):
  File "/home/user2/.venv/experiment_with_shared.py", line 17, in <module>
    dataset = datasets.load_dataset(DATASET)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user2/.venv/lib/python3.12/site-packages/datasets/load.py", line 1397, in load_dataset
    builder_instance = load_dataset_builder(
                       ^^^^^^^^^^^^^^^^^^^^^
  File "/home/user2/.venv/lib/python3.12/site-packages/datasets/load.py", line 1171, in load_dataset_builder
    builder_instance: DatasetBuilder = builder_cls(
                                       ^^^^^^^^^^^^
  File "/home/user2/.venv/lib/python3.12/site-packages/datasets/builder.py", line 390, in __init__
    with FileLock(lock_path):
  File "/home/user2/.venv/lib/python3.12/site-packages/filelock/_api.py", line 377, in __enter__
    self.acquire()
  File "/home/user2/.venv/lib/python3.12/site-packages/filelock/_api.py", line 333, in acquire
    self._acquire()
  File "/home/user2/.venv/lib/python3.12/site-packages/filelock/_unix.py", line 45, in _acquire
    fd = os.open(self.lock_file, open_flags, self._context.mode)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied: '/models/hf_hub_shared_experiment/data/_models_hf_hub_shared_experiment_data_tatsu-lab___alpaca_default_0.0.0_dce01c9b08f87459cf36a430d809084718273017.lock'
```

### Expected behavior

The second user should be able to read the shared cache files.

### Environment info

$ datasets-cli env

- `datasets` version: 4.4.1
- Platform: Linux-6.8.0-88-generic-x86_64-with-glibc2.39
- Python version: 3.12.3
- `huggingface_hub` version: 0.36.0
- PyArrow version: 22.0.0
- Pandas version: 2.3.3
- `fsspec` version: 2025.10.0

Comments:
I remember a fix from last year to usr the current umask for filelock 3.10.0, which filelock version are you using ? can you try another version ?

I believe we we are using `filelock==3.19.1`. Do you have a recommended version to use?

Our test suite has been testing all versions over time for years now, including 3.19.1. And we didn't get failures for our tests.

I'm not sure but this could be an actual permission error unrelated to filelock then