ensure no any user writable permissions in Nextcloud sync folder

ensure no any user writable permissions in Nextcloud sync folder

past versions may have wrongly set the write permissions for other users
(UNIX style permissions)

remove this under the hypothesis that newly created files or folders
gets more restrictive permissions and that past files or folders should
be updated to get the same permissions

Signed-off-by: Matthieu Gallien <matthieu.gallien@nextcloud.com>