Always use the same Debian mirror for time-based APT snapshots

Originally created by @intrigeri on #12454 (Redmine)

We often see warnings like this one:

Warning: downgrading 'va-driver-all' from '1.8.1-1' to '1.8.0-1' in 'experimental|main|i386'!

I think this can cause all sorts of confusion when analyzing build & test results, and would be avoided if we always used the exact same mirror when pulling from the Debian archive.

Related issues

• Related to tails/sysadmin#13134