bug#38795: "guix build" doesn't find modules from extra channels when building from an expression

[pkill9]

I have added my own repository to my ~/.config/guix/channels.scm 
(https://gitlab.com/pkill-9/guix-packages-free), but when I run `guix build 
--expression='(use-modules (pkill9 utils))' it returns "no code for module 
(pkill9 utils)".

It does find modules from the guix repository though, e.g. `guix build 
--expression='(use-modules (guix packages))` returns "guix build: error: 
#<unspecified>: not something we can build" without a "no code for module" 
error.