* Committing anything to vagrant/definitions/tails-builder means there
  is a new version of the builder VM (and it will automatically be
  generated next Tails build)
* Separately track the time-based snapshot serials used to generate
  the builder

These changes makes it extremely convenient to release new boxes (just
commit) and avoids issues where commits made to
vagrant/definitions/tails-builder may affect *some* users base box
generation, but not others, depending on when they Git fetch (see
ticket #12409, comment 32).

Will-fix: #12409