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.

src/nspawn/nspawn.c

@@ -2278,7 +2278,7 @@ static int inner_child(
                         return log_error_errno(errno, "Failed to change to specified working directory %s: %m", arg_chdir);
 
         if (arg_start_mode == START_PID2) {
-                r = stub_pid1();
+                r = stub_pid1(arg_uuid);
                 if (r < 0)
                         return r;
         }