tox: Add find command to allowlist_externals list

whitelist_externals is deprecated now, use allowlist_externals
instead. see[0]
Add find command to allowlist_externals list to fix a warning.[1]

[0]: https://tox.readthedocs.io/en/latest/config.html
[1]: https://zuul.opendev.org/t/openstack/build/9ac7f205080343f29d7e2b519abbdb73/log/job-output.txt#945

Change-Id: I67e9c892a945d04eff59427a0161aafceb136560