Avoid symlink() error if acme conf already exists as a file.

See https://www.howtoforge.com/community/threads/error-installing-update-to-3-2-7p1.88618/#post-434045

Likely the 'PHP Warning: symlink(): No such file or directory in /tmp/update_runner.sh.5heAlRJWup/install/lib/installer_base.lib.php on line 2879' came from them having a 999-acme.vhost file (not symlink) already