bug#61099: guix shell '-E' option can be silently ignored

[Maxim Cournoyer]

Hello,

I just encountered a situation where -E is not correctly parsed/used and
ignore silently.

Reproducer:

--8<---------------cut here---------------start------------->8---
guix shell -CD igraph -E /gnu=/gnu strace
--8<---------------cut here---------------end--------------->8---

The above doesn't work (there are far too few entries to /gnu/store, it
isn't shared).

This works:

--8<---------------cut here---------------start------------->8---
guix shell -CD igraph --expose=/gnu=/gnu strace
--8<---------------cut here---------------end--------------->8---

-- 
Thanks,
Maxim