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

[pkill9 <pkill9@runbox.com>]

Running the following in `guix repl` returns additional channels:
```
itsme@antelope ~> guix repl
GNU Guile 3.0.4
Copyright (C) 1995-2020 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guix-user)> (use-modules (gnu packages))
scheme@(guix-user)> (%package-module-path)
$1 =
(("/gnu/store/kds0mq06qpin125gkikwzdm6mjfwjffc-guix-module-union/share/guile/site/3.0"
. "gnu/packages")
"/gnu/store/96pa4rc57zgqf36y2kv8z20p2jvlgypq-pkill9-free-channel-dependency/share/guile/site/3.0"
"/gnu/store/3ilx18ywdm6xk9f5l1mznrn45vcbncsq-pkill9-free/share/guile/site/3.0")
scheme@(guix-user)>
```

But running the following in "test.scm" with `guix repl /tmp/test.scm
doesn't return additional channels:
```
(use-modules (gnu packages))
(display (%package-module-path))
```

```
((/gnu/store/kds0mq06qpin125gkikwzdm6mjfwjffc-guix-module-union/share/guile/site/3.0
. gnu/packages))
```

fold-available-packages uses this to search for packages, which I am
using for a script. As a result, the script doesn't know about packages
from the additional channels.