nspawn: flush out environment block of the -a stub init process

nspawn: flush out environment block of the -a stub init process

The container detection code in virt.c we ship checks for /proc/1/environ,
looking for "container=" in it. Let's make sure our "-a" init stub exposes that
correctly.

Without this "systemd-detect-virt" run in a "-a" container won't detect that it
is being run in a container.