Fixes a regression introduced by
4935501fd9 that would skip generating
the user's ~/.config/zsh/.zshenv unless both of zshenv and
environment-variables are set in the configuration.
* gnu/home/services/shells.scm (zsh-get-configuration-files):
Generate ~/.config/zsh/.zshenv when only one of zshenv or
environment-variables is specified.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>