accounts: Always honor the configured user account shell.

Starting from commit 0ae735bc, Guix
System would preserve the user shell across reconfigure and reboot.
This was done so as to allow for the use of 'chsh'.

This proved to be a misguided decision.  This commit goes back to
considering user shells as config and not "state."

* gnu/build/accounts.scm (allocate-passwd): Do not use shell from
PREVIOUS.