community/cni-plugin-flannel: fix binary filename

When running k3s, the pods that get deployed are looking for /usr/libexec/cni/flannel, not /usr/libexec/cni/flannel-amd64.

Originally posted as a review comment here: !38444 (comment 259629)

As a temporary fix, I can create a symlink: ln -s flannel-amd64 flannel