bug#42688: Running a script with `guix repl` doesn't "see" additional channels using (%package-module-path)

[Leo Prikler]

Hi Guix,

I've finally figured out, what causes this issue.

Guix repl uses the following code to call scripts:
```
    (unless (null? script)
      ;; Run script
      (save-module-excursion
       (lambda ()
         (set-program-arguments script)
         (set-user-module)
         (load-in-vicinity "." (car script)))))
```

But `guix describe` (which is used to initialize %package-module-path)
has the following:

```
(define current-profile
  (mlambda ()
    "Return the profile (created by 'guix pull') the calling process
lives in,
or #f if this is not applicable."
    (match (command-line)
      ((program . _)
       (and (string-suffix? "/bin/guix" program)
           [...])))))

(define current-profile-entries [...])
(define current-channel-entries [...])
(define package-path-entries [...])
```

Each of these procedures depends on the previous, building up a chain
that fails exactly in the case where we (set-program-arguments [...])
with a script other than the current channel's guix (which is probably
the way you'd want to use `guix repl`).  

There are some ways of resolving this.  One would be to access earlier
versions of "command-line" – it does resolve to a fluid, but that fluid
itself is not exposed to Guile.  Perhaps there might be some FFI magic
to access it.

You could also set up your script to fake being a Guix command by
setting the command line to be (cons*
"$HOME/.config/guix/current/bin/guix" "repl" (command-line)), i.e.
reconstructing the way your script has been invoked.  This would
obviously break if you were to call it with a different Guix, also
you'd have to resolve $HOME instead of writing it like that, but you'd
have access to your channels.

On the other hand, we could patch `guix repl` to initialize %package-
module-path earlier (still leaving `guix describe` broken) or somehow
try to work around that issue in `guix describe`.

Regards,
Leo