Do not serialize environment, when switching root

When switching root, i.e. LANG can be set to the locale of the initramfs
or "C", if it was unset. When systemd deserializes LANG in the real root
this would overwrite the setting previously gathered by locale_set().

To reproduce, boot with an initramfs without locale.conf or change
/etc/locale.conf to a different language than the initramfs and check a
daemon started by systemd:

$ tr "$\000" '\n' </proc/$(pidof sshd)/environ | grep LANG
LANG=C

To prevent that, serialization of environment variables is skipped, when
serializing for switching root.

https://bugzilla.redhat.com/show_bug.cgi?id=949525