Fix permissions on /run/systemd/nspawn/locks

Fix permissions on /run/systemd/nspawn/locks

machined is getting an EACCES when trying to create the lock file for
images because the mode on /run/systemd/nspawn/locks is 0600.

mkdir("/run/systemd/nspawn/locks", 0600) = -1 EEXIST (File exists)
stat("/run/systemd/nspawn/locks", {st_mode=S_IFDIR|0600, st_size=40, ...}) = 0
open("/run/systemd/nspawn/locks/inode-41:256", O_RDWR|O_CREAT|O_NOCTTY|O_NOFOLLOW|O_CLOEXEC, 0600) = -1 EACCES (Permission denied)

This commit adjusts the mode to 0700 to correct the issue.