bug#71299: Better handle "updating substitutes" messages in CI pipelines and dumb terminals

[Richard Sent <richard@freakingpenguin.com>]

Hi Guix!

Due to it's nature as an incremental counter (from 0% to 100%), Guix
prints many messages to the terminal when it is updating substitutes.
This hurts CI log files because large portions of the output is filled
with cruft [1]. This problem is made worse because Guix updates
substitutes multiple times during a build.

verbosity=0 can be used to disable ALL output messages. However, this
isn't ideal for automated build pipelines where people need to monitor
the status and review what may have gone wrong.

Because "updating substitutes" prints so many lines to the terminal that
aren't typically relevant from a log reviewer's perspective that hide
useful information, it should be possible to disable just those
messages.

Alternatively, I believe Guix has code to detect when it's attached to a
"dumb" terminal. Perhaps these messages should be conditionally removed
or replaced with a pair of static messages like the following:

--8<---------------cut here---------------start------------->8---
substitute: Updating subsitutes from .....
substitute: Finished updating substitutes from ....
--8<---------------cut here---------------end--------------->8---

This would make log files much easier to parse. Instead of having
potentially thousands of lines printed to the console per "build stage",
it would be limited to 2.

[1]: https://builds.sr.ht/query/log/1238029/update-readme/log
-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.