Change DAVE to check for amd64 IDF
Once we ship an amd64 ISO only, we should change DAVE to download the IDF on
https://tails.boum.org/install/v1/Tails/amd64/stable/latest.yml instead of
Apparently the needed change is a mere s/i386/amd64/ in conf.json.
This should happen after 3.0.