prepare_module issue when loading from read-only fs
## Describe the bug

When we use prepare_module from a readonly file system, we create a FileLock using the `local_path`.
This path is not necessarily writable.

`lock_path = local_path + ".lock"`


## Steps to reproduce the bug

Run `load_dataset` on a readonly python loader file.
```python
ds = load_dataset(
        python_loader, data_files={"train": train_path, "test": test_path}
    )
```

where `python_loader` is a path to a file located in a readonly folder.

## Expected results
This should work I think?

## Actual results

```python
    return load_dataset(
  File "/usr/local/lib/python3.8/dist-packages/datasets/load.py", line 711, in load_dataset
    module_path, hash, resolved_file_path = prepare_module(
  File "/usr/local/lib/python3.8/dist-packages/datasets/load.py", line 465, in prepare_module
    with FileLock(lock_path):
  File "/usr/local/lib/python3.8/dist-packages/datasets/utils/filelock.py", line 314, in __enter__
    self.acquire()
  File "/usr/local/lib/python3.8/dist-packages/datasets/utils/filelock.py", line 263, in acquire
    self._acquire()
  File "/usr/local/lib/python3.8/dist-packages/datasets/utils/filelock.py", line 378, in _acquire
    fd = os.open(self._lock_file, open_mode)
OSError: [Errno 30] Read-only file system: 'YOUR_FILE.py.lock'
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.7.0
- Platform: macOS-10.15.7-x86_64-i386-64bit
- Python version: 3.8.8
- PyArrow version: 3.0.0
