Exclude systemd from updates

The systemd versions ~>254 have changed the locking mechanism they use
for updated /etc/passwd and likely other files. They are hard-coded
now to use the Linux-specific fcntl(F_OFD_SETLKW) and removed the flock() fallback
https://github.com/systemd/systemd/pull/26726/commits/ed70a34ec0f9ac53646101dd9c544b1f60d17e69
However, not all container runtime support it
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1053749

Fixes #86 (closed)