schnapps: prevent execution on non-btrfs filesystems

We check if we are running on current root but we have to also check if we are running on BTRFS because otherwise it makes no sense to run schnapps. Even if schnapps would be configured to work on some other filesystem in terms of updater integration that is kind of useless. The files are stored primarily on root and thus root is what we care about.

This is fix for updates taking looong time on nsfarm because of broken mmc. We are not running on btrfs and thus it makes no sense to run schnapps.