disable IPv6 in apt.lizard

Time-based APT snapshots of debian-security have been failing, the probable cause being instability in the connection between our colo and Fastly.

Unfortunately, official mirrors for debian-security seem to be all behind Fastly, and Debian doesn't encourage the use of unofficial debian-security mirrors:

Q: Why are there no official mirrors for security.debian.org?

A: Actually, there are. There are several official mirrors, implemented
through DNS aliases. The purpose of security.debian.org is to make
security updates available as quickly and easily as possible.

Encouraging the use of unofficial mirrors would add extra complexity
that is usually not needed and that can cause frustration if these
mirrors are not kept up to date.

https://www.debian.org/security/faq#mirror

Weirdly, and even though we don't use IPv6, looks like having it enabled in the kernel could be a trigger for this issue, so we disable it using sysctl.

refs tpo/tpa/tails-sysadmin#18197