units: pull in basic.target rather than sysinit.target from system-install.target

units: pull in basic.target rather than sysinit.target from system-install.target

Many of our services are nowadays implemented via socket activation, and
hence require sockets.target to be active to be accessible. One of them
is mute-console.socket, which we typically want to use from
systemd-firstboot.service, systemd-sysinstall.service and other related
services. Hence let's pull in basic.target rather than sysinit.target
from system-install.target since it pulls sockets.target in too.

Effectively, this doesn't change much except for pulling in a bunch more
sockets, and frankly going for sysinit.target was really a bug to begin
width.