Hi Guix!

When updating my system, I noticed there was a duplicate php-fpm group.
The reason is that by default, the group is php-fpm, and the accounts
list contains php-fpm and the configured group. I'm not sure why, but
the hardcoded php-fpm group seems to be used by the user as a
supplementary group.

This patch only adds the php-fpm group when the configured group is not
already php-fpm. If you think it'd be better, we could also simply
remove the hard-coded php-fpm group and the supplementary group from
the php-fpm user.