bug#74711: Sharing some progress

[ngraves]

I've found the reason behind the extremely annoying SSL certification 
refusals.

When I use an installation image, the date is not necessarily set at the 
real date.  In my case, `date` was set in 2019, and triggered the SSL 
verification refusal (not yet valid).

I don't know why it sometimes doesn't happen.  If we can't fix/automate 
it at the time we boot in the installation medium, we should probably 
add a warning in the manual / a hint in guix pull / a proper error in 
guile-git (that could provide more information than just Git failing) ?

WDYT ?

Best regards,
Nicolas