But this does not help with the cookbook’s guile channel nor Nigko’s
test-repo, even after repeatedly pulling a guix channel with this
change.

I might misunderstand .go compilation and whether macros are evaluated
at compile-time.  But maybe the error is because `local-file' is
relative to the current-source-directory from which the .go file was
compiled.  But the .go file was compiled from a directory union that
does not contain the source code, it only has the modules directory
listed in .guix-channel.  Only when the pre-compiled .go is not used,
the source directory can be found.  (Am I on the right track or
mistaken?)

/var/guix/profiles/per-user/florian/current-guix links to it.  guix gc
--referrers and --derivers show what is built from what.  Maybe I
misunderstand.  Maybe the channel build system would need changes.

In particular,

$ guix build -S guile@3.0.99-git

never works no matter the load path.

Regards,
Florian