Nightly ISO images are regularly unavailable for a couple minutes

Originally created by @intrigeri on #11557 (Redmine)

Reference: https://icingaweb2.tails.boum.org/monitoring/service/history?host=ecours.tails.boum.org&service=nightly_devel

IIRC I’ve reported this bug during the initial dev phase of the monitoring system, and we decided it wasn’t a bug in the monitoring system but in the actual service. So let’s fix the bug in the actual service then :)

Today the 1st occurrence of this problem in www.lizard nginx’ error.log is at 07:54:08. Yesterday it was at 07:54:17. On July 2 it was at 07:54:26, etc. I doubt that’s a coincidence, and I suspect we have a cronjob or something that breaks this stuff everyday or close to it (and then either our monitoring is fast enough to report it to us, or it’s not).