Fix ANTIVIRUS line in mailu.env file

The ANTIVIRUS line is commented in mailu.env file. Each mailu.env file generated by setup.mailu.io will have the antivirus line commented. Removed the # so that antivirus works again for new deployments.

setup/flavors/compose/mailu.env

@@ -51,7 +51,7 @@ WEBMAIL={{ webmail_type }}
 WEBDAV={{ webdav_enabled or 'none' }}
 
 # Antivirus solution (value: clamav, none)
-#ANTIVIRUS={{ antivirus_enabled or 'none' }}
+ANTIVIRUS={{ antivirus_enabled or 'none' }}
 
 ###################################
 # Mail settings