byzantium: Switch to https

There's a workaround in place on the archive server so that this
can work again.

conf/dists/byzantium

-mirror="http://repo.pureos.net/pureos"
-apt_sources="deb http://repo.pureos.net/pureos byzantium main
-deb http://repo.pureos.net/pureos byzantium-updates main
-deb http://repo.pureos.net/pureos byzantium-security main
+mirror="https://repo.pureos.net/pureos"
+apt_sources="deb https://repo.pureos.net/pureos byzantium main
+deb https://repo.pureos.net/pureos byzantium-updates main
+deb https://repo.pureos.net/pureos byzantium-security main
 "
 debootstrap_dist=byzantium