apt-snapshots partition lacks disk space

Originally created by @intrigeri on #13526 (Redmine)

On tails/sysadmin#12725 we temporarily addressed the problem, but we’re back to a problematic situation: only 27GB available today. We still have a little bit of margin (75GB on SSDs, 50GB on rotating drives, better not use those for this service) to allocate more space though. I’ll check if we can do that without harming other ever-growing services.

Related issues

• Related to tails/sysadmin#13425
• Related to tails/sysadmin#12725
• Related to #13579 (closed)
• Related to tails/sysadmin#11912
• Blocks tails/sysadmin#13233