The Vagrant VM has too little memory for disk builds

As discovered on #12741 (closed):

• url : https://gitlab.com/arnaud-preevio/tails.git
• branch: arnaud/bump-vm-memory

Feature Branch: bugfix/13480-bump-build-VM-memory

Related issues

• Blocks #12741 (closed)

Original created by @intrigeri on 13480 (Redmine)