This branch already assumes it to be the case in some places, and it's
handy since with an actual checkout, we can use scripts, like
auto/scripts/apt-snapshots-serials, that we need. We also adapt the
build-tails script to this change.

(Note: This change is actually already present in another
branch, wip/11972-use-vagrant-in-jenkins branch, but it's not suitable
for merging into this one yet.)