Static CNI bin dir not usable in k3s docker image

[brandond]

Is your feature request related to a problem? Please describe.

We added a static CNI bin dir in #10868, but that directory cannot be used in the K3s docker image because the CNI bins in the docker image are just in /bin:

root@k3s-server-1:~# grep -C1 bin_dir /var/lib/rancher/k3s/agent/etc/containerd/config.toml
[plugins."io.containerd.grpc.v1.cri".cni]
  bin_dir = "/bin"
  conf_dir = "/var/lib/rancher/k3s/agent/etc/cni/net.d"

root@k3s-server-1:~# /bin/cni --version
CNI plugins plugin v1.6.0-k3s1
CNI protocol versions supported: 0.1.0, 0.2.0, 0.3.0, 0.3.1, 0.4.0, 1.0.0, 1.1.0

Describe the solution you'd like

The static CNI bin dir /var/lib/rancher/k3s/data/cni should also be usable in the k3s docker image.

We could possibly just symlink /var/lib/rancher/k3s/data/cni to /bin and place it first in the search path? Needs more investigation.

Describe alternatives you've considered

Cover this difference in behavior in the docs.

Additional context