Freeze APT snapshots at 2019080601 for Tails 4.0~beta1.

The current snapshots are actually at 2019080701 but I have seen FTBFS
on Jenkins (compile error for virtualbox-guest) and a different one
locally (Tor Browser AppArmor profile patch doesn't apply) so I'll
pick the most recent snapshots I successfully built to avoid having to
delay the release further.