Upgrade Lizard's memory again

The amount of memory needed for ram build has been bumped in the feature/stretch branch, which lead to build failure in our Jenkins setup when using vagrant, as the isobuilders don’t have enough configured memory to cope.

When we’ll release Tails based on Stretch and switch all our isobuilders to the vagrant build, we’ll certainly need to add more memory to Lizard if we want to keep the same amount of iso{builders,testers}.

Parent Task: #11972 (closed)

Original created by @bertagaz on 12327 (Redmine)