Delete the Vagrant VM used for an ISO build once it is finished
_Originally created by @intrigeri on [#11981 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/11981)_
Given tails/tails#11980, this is needed to avoid wasting lots of disk space.
Feature Branch: wip/11972-use-vagrant-in-jenkins
Parent Task: tails/tails#5630
### Related issues
- [x] **Blocked by** tails/tails#11980
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