Fixes https://github.com/owncloud/core/issues/13657

There is not much sense in having these files marked executable, we should avoid that.

It isn't uncommon that admins create a backup file of the config (i.e. `config.php.bak`) before performing any changes. This would allow everybody to read the backup of the configuration file which contain several secret and critical values.

I don't believe this is worth a backport or getting added to the installer. It's just a nice to have. People that create public readable backups of their configuration are the one to blame, not us :-)