bug#57775: guix shell --root fails

[Ricardo Wurmus]

This fails on the first try:

--8<---------------cut here---------------start------------->8---
$ guix shell -D -f guix.scm --root=env-test
Backtrace:
          10 (primitive-load "/home/rekado/.config/guix/current/bin/guix")
In guix/ui.scm:
   2238:7  9 (run-guix . _)
  2201:10  8 (run-guix-command _ . _)
In ice-9/boot-9.scm:
  1752:10  7 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In guix/status.scm:
    815:4  6 (call-with-status-report _ _)
In guix/scripts/environment.scm:
   946:13  5 (_)
In guix/store.scm:
  2168:25  4 (run-with-store #f #<procedure 7fb5e6f7fe60 at 
guix/scripts/environment.scm:948:14 (state)> #:guile-for-build _ #:system _ # _)
In guix/scripts/environment.scm:
   963:16  3 (_ _)
In guix/store.scm:
  2040:38  2 (_ #f)
   1475:0  1 (add-indirect-root #f "/home/rekado/dev/pigx/pigx_bsseq/env-test")
In ice-9/boot-9.scm:
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure struct-vtable: Wrong type argument in position 1 (expecting 
struct): #f
--8<---------------cut here---------------end--------------->8---

On the second try it does work and links the profile to ./env-test.

-- 
Ricardo