ceph-ansible-prs: pin tox version

ceph-ansible-prs: pin tox version

Due to a regression introduced in tox 4.3.1, multiple environment variable substitution
is broken.
```
tox.report.HandledError: replace failed in centos-non_container-update.commands with MatchRecursionError('circular chain between set env DOCKER_HUB_USERNAME, DOCKER_HUB_PASSWORD, INVENTORY')
```

Let's pin to the last tox stable version where it used to work fine (4.2.8).

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>