GitLab CI is super slow and APT operations timeout

see e.g.

• currently running, super slow https://gitlab.tails.boum.org/tails/tails/-/jobs/186149
• failures to connect to deb.debian.org: https://gitlab.tails.boum.org/tails/tails/-/jobs/186097
• timeout https://gitlab.tails.boum.org/tails/tails/-/jobs/186094
• pipeline runs that take 2 hours: https://gitlab.tails.boum.org/tails/tails/-/pipelines/16764, https://gitlab.tails.boum.org/tails/tails/-/pipelines/16767

(on Thursday we noticed that with @zen and I thought let's see what's up on Monday. here we are.)