bug#71298: --verbosity=0 does not disable all substitute messages in shell

[Richard Sent]

Hi Guix!

Setting verbosity to 0 (which according to the manual disables all
output) does not actually disable all output in Guix shell.

--8<---------------cut here---------------start------------->8---
~/code/projects/srht-readme $ guix shell --verbosity=0 ghc -- ls
substitute: updating substitutes from 'http://10.1.2.2:80'... 100.0%
substitute: updating substitutes from 'https://bordeaux.guix.gnu.org'... 100.0%
COPYING  org-to-html  README.html  README.org  setup-environment
--8<---------------cut here---------------end--------------->8---

--8<---------------cut here---------------start------------->8---
~/code/projects/srht-readme $ guix shell emacs-back-button --verbosity=0
substitute: updating substitutes from 'http://10.1.2.2:80'... 100.0%
substitute: updating substitutes from 'https://bordeaux.guix.gnu.org'... 100.0%
gibraltar :) srht-readme [env]$ 
--8<---------------cut here---------------end--------------->8---

This problem seems unique to guix shell.

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.