bug#67224: [Cuirass] ‘remote-server’ exits due to “Host unreachable”

[Ludovic Courtès]

‘cuirass remote-server’ (as of 1.2.0) exits on “Host unreachable” errors
from ZeroMQ:

--8<---------------cut here---------------start------------->8---
2023-11-16 09:17:59 periodic update: 0 resumable, 0 failed builds
2023-11-16 09:18:02 141.80.167.161 (EeiKw8YX): build started: 
'/gnu/store/dvxb3sf6aakriw8554pavazyd84mcgd5-r-singler-2.4.0.drv'.
2023-11-16 09:18:37 error: ZeroMQ error in build server: Host unreachable
2023-11-16 09:18:37 increased maximum number of open files from 1024 to 2048
2023-11-16 09:18:37 listening for build logs on port 5556
2023-11-16 09:18:37 deleting old build logs from '/var/cache/cuirass/remote/'...
2023-11-16 09:18:52 selected 0 build logs to remove
2023-11-16 09:18:52 WARNING: (cuirass scripts remote-server): imported module 
(fibers) overrides core binding `sleep'
2023-11-16 09:18:52 starting fetch worker with up to 8 concurrent downloads
2023-11-16 09:19:20 error: ZeroMQ error in build server: Host unreachable
--8<---------------cut here---------------end--------------->8---

This in turn occasionally triggers this ‘remote-worker’ bug:
<https://issues.guix.gnu.org/66980>.  Which in turn can trigger this
one, and so on.

Ludo’.