Add ability to set lock type
We've replaced our filelock implementation with the `filelock` package, so their repo is the right place to request this feature.

In the meantime, the following should work: 
```python
import filelock
filelock.FileLock = filelock.SoftFileLock

import datasets
...
```
### Feature request

Allow setting file lock type, maybe from an environment variable

Currently, it only depends on whether fnctl is available:

https://github.com/huggingface/datasets/blob/12ebe695b4748c5a26e08b44ed51955f74f5801d/src/datasets/utils/filelock.py#L463-L470C16

### Motivation

In my environment, flock isn't supported on a network attached drive

### Your contribution

I'll be happy to submit a pr.