bug#36509: guix pull: too many substitute updates, or bad log messages

[Robert Vollmert]

When running `guix pull`, output like the following is common
for me:

$ guix pull
Updating channel 'puzzlink' from Git repository at '/home/rob/guix-puzzlink'…
[…]
Building from these channels:
  guix      https://github.com/robx/guix.git    11f68b3
[…]
Computing Guix derivation for 'x86_64-linux'... |
The following derivations will be built:
   /gnu/store/ym0l8rvnh3aa3qv8g3xgh1mszbhww8c2-elm.drv
[…]
   /gnu/store/dcxwfh37bn8r3h8s9w7wfq8yhzynqxx5-profile.drv
The following profile hooks will be built:
   /gnu/store/biw1n2a956skpaa4m7rmka6j8h1clhmv-guix-package-cache.drv
[…]
   /gnu/store/kxrkynck33ypwyy09ypi2qm7737fsw28-fonts-dir.drv
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
building /gnu/store/wxfi8mdp11l5k4ngkbsy9apcg9h74jfk-guix-system.drv…

Either guix is making useless extra downloads or:

(a) the substitute log message should be changed to differentiate between
the different calls
(b) there should be extra log messages before the substitute messages to
indicate what guix is doing that causes the individual substitue messages.