main: ISOLATE rather than REPLACE default.target

This allows switch-root to work correctly if a unit is active both before and
after the switch-root, but its dependencies change. Before the patch, any
dependencies added to active units by switch-root will not be pulled, in
particular filesystems configured in /etc/fstab would not be activated if
local-fs.target was active in the initrd.

It is not clear to me if there is a bug in the REPLACE handling, or if it is
working as expected and that we really want to use ISOLATE instead as this patch
does.