Add an OpenPGP key on builder.lizard to sign Jenkins artifacts

For the jenkins artifacts to be published on nightly.tails.boum.org in a bit more safe manner, we need to have them signed at some point. We can add a gnupg key to the jenkins user on builder.lizard, managed by puppet like the one of our apt repo.

Parent Task: #6193 (closed)

Original created by @bertagaz on 6266 (Redmine)