Ensure image-builder continues to work once kubernetes-cni is discontinued

[sbueringer]

There is an ongoing discussion about discontinuing the kubernetes-cni package (kubernetes/release#4156).

Related discussion here: kubernetes/release#4100

IIRC image-builder has a dependency on it, i.e. it always tries to install it (not sure if it is actually required to install the package)

[BenTheElder]

FWIW you could basically just grab the upstream CNI tarball, the packages a little more than re-tarring that binary tarball into deb/RPM format.

YMMV on if you need it, depends on who is responsible for configuring CNI and what they're using.