Use full path for certbot-auto in cronjob

On my Debian 10 system, the cronjob for certbot-auto fails because it can't find certbot-auto. It looks like the default environment used for root's crontab is pretty sparse and doesn't include /usr/local/bin, so I've changed the cronjob to refer to certbot-auto by its full path, which fixes the issue and is safer in general.