acme: Use `chown -R` for challenges directory. Fixes #24529.

Commit 75f131da added
`chown 'nginx:nginx' '/var/lib/acme'` to the pre-start script,
but since it doesn't use `chown -R`, it is possible that there
are older existing subdirs (like `acme-challenge`)
that are owned to `root` from before that commit went it.