Fix debian releases w/o updates and security fixes
Fix debian releases w/o updates and security fixes

Actually Debian testing release has security updates available, metadata
are available at these URL:

http://ftp.debian.org/debian/dists/testing-updates/
http://security.debian.org/debian-security/dists/testing-security/

However, sid (the other name of unstable) does not have additional
security updates repositories, as fixes land in sid main repository
directly.

systemd/mkosi,systemd/mkosi