environment: Fix ‘--check’ with exported PS1 variable.

environment: Fix ‘--check’ with exported PS1 variable.

If your .bashrc (unnecessarily) exports PS1 and you’re in a non-login
shell, Guix crashes:

user@popigai:~$ guix shell --check coreutils
guix shell: checking the environment variables visible from shell '/bin/bash'...
Backtrace:
          14 (primitive-load "/home/user/.config/guix/current/bin/g…")
In guix/ui.scm:
   2206:7 13 (run-guix . _)
  2169:10 12 (run-guix-command _ . _)
In ice-9/boot-9.scm:
  1752:10 11 (with-exception-handler _ _ #:unwind? _ # _)
  1752:10 10 (with-exception-handler _ _ #:unwind? _ # _)
In guix/store.scm:
   658:37  9 (thunk)
   1320:8  8 (call-with-build-handler _ _)
   1320:8  7 (call-with-build-handler #<procedure 7fe393b7c420 at g…> …)
In guix/status.scm:
    800:4  6 (call-with-status-report _ _)
In guix/scripts/environment.scm:
   951:12  5 (_)
In guix/store.scm:
  2119:24  4 (run-with-store #<store-connection 256.99 7fe39627e500> …)
In guix/scripts/environment.scm:
   521:35  3 (_ _)
In unknown file:
           2 (string=? ("PS1" . "${debian_chroot:+($debian_chroot)…") …)
In ice-9/boot-9.scm:
  1685:16  1 (raise-exception _ #:continuable? _)
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure string=: Wrong type argument in position 1 (expecting string): ("PS1" . "${debian_chroot:+($debian_chroot)}\\[\\033[01;32m\\]\\u@\\h\\[\\033[00m\\]:\\[\\033[01;34m\\]\\w\\[\\033[00m\\]${EXTRA}\\$ ")
user@popigai:~$

This is because the match pattern doesn’t expect `(vhash-assoc "PS1" actual)`
to return a pair.

* guix/scripts/environment.scm (validate-child-shell-environment): Change
match pattern to expect a pair.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>