In a few minutes, we’ll make 12.0.0RC2 available via the updater app.
There may be a small issue with the updater in previous versions. If your Nextcloud built-in updater is not working because of the extra file autotest-checkers.sh then you need to manually delete the file autotest-checkers.sh from your Nextcloud folder. (e.g. /var/www/html/nextcloud/)