core: do not attempt to add 'private' symlinks when RootImage/RootDirectory are used

core: do not attempt to add 'private' symlinks when RootImage/RootDirectory are used

A bind mount is added directly from private on the host to the actual
destination directory, no need for the symlinks (which cannot be created
as the bind mount happens first and creates the target as an actual directory)

Fixes https://github.com/systemd/systemd/issues/22264

(cherry picked from commit 3fa80e5e75a98ef6f9a84b01770b71a1774478dc)

Conflicts:
	test/units/testsuite-50.sh