bug#56319: 'guix system' failure due to substitute being temporarily 404 -> Implement a retry attempts?

[Maxime Devos]

merge 56005 56320 56319
thanks

This doesn't look Guix System-specific to me and looks to have the same issue 
(i.e., something about write_wait_fd) as
<https://issues.guix.gnu.org/56005>, so merging into 56005.

Also, is the substitute server being down a hypothesis for the error but not 
actually known to actually have hapend, or something you have noticed the 
substitute server being down via other methods and believe it to be a plausible 
hypothesis for the error cause?

In all cases, I believe write_wait_fd: unimplemented to be a bug that needs to 
be investigated for the root cause and fixed, not something to accumulate 
work-arounds for and ignore by automatic retries.

Greetings,
Maxime.