Unable to set file as unreadable inside tox container

I have a Python module that relies on setting permissons to a file as unreadable.

Unfortunately, it looks like this is not possible inside the tox container.

Here's the minimal reproducer : https://github.com/matthew-brett/test-actions/blob/main/tox.ini

In particular, I have this test in my `tox.ini` file:

```
touch unreadable
python -c "import os; os.chmod('unreadable', 0o200)"
python -c "open('unreadable', 'rb').read()"
```

Obviously the last line should cause a `Permission denied` error, and that is what happens locally. But running via this action using [this minimal workflow](https://github.com/matthew-brett/test-actions/blob/main/.github/workflows/test.yml), I get no error - Python can read the unreadable file.

I wonder whether this is because the tox tests are running as some privileged user, for whom setting the user read permission is not effective?