bug#64581: ‘--nesting’ option of ‘guix shell’ has no effect when combined with ‘-f’

[Ludovic Courtès]

In the first case below, there’s no ‘guix’ command in the environment,
whereas in the second case there is one, as expected:

--8<---------------cut here---------------start------------->8---
$ guix shell -CPW -D -f guix.scm -- guix
guix shell: error: guix: command not found
hint: Did you mean 'guild'?

$ guix shell -CPW -D coreutils -- guix
guix: missing command name
Try `guix --help' for more information.

$ guix describe
Generation 269  Jul 10 2023 00:17:51    (current)
  guix d2cf631
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: d2cf63165119fc78c9f52abce2ce66af23e49243
--8<---------------cut here---------------end--------------->8---

Ludo’.