bug#48564: ‘channel-with-substitutes-available’ makes assumptions about job names

[Mathieu Othacehe]

Hey Ludo,

> Wouldn’t it be enough to look for the latest completed evaluation (of a
> given jobset)?

I guess it would be enough but we would need to specify the jobset that
needs to be used, this way maybe:

--8<---------------cut here---------------start------------->8---
(define* (channel-with-substitutes-available chan url
                                             #:key
                                             (specification "master"))
--8<---------------cut here---------------end--------------->8---

this would also allow me to simplify this patchset:
https://issues.guix.gnu.org/47929.

WDYT?

Thanks,

Mathieu