Add nft to k3s-root userspace bundle

[brandond]

The NFTablesProxyMode feature-gate is beta and on by default in Kubernetes v1.31:

• https://kubernetes.io/blog/2024/08/13/kubernetes-v1-31-release/#nftables-backend-for-kube-proxy
• https://kubernetes.io/docs/reference/networking/virtual-ips/#proxy-mode-nftables

When started with --kube-proxy-arg=proxy-mode=nftables, kube-proxy will look for the nft binary. If it is not found, kube-proxy will exit and k3s will fail to start:

ERRO[0003] kube-proxy exited: unable to create proxier: unable to create ipv4 proxier: could not find nftables binary: exec: "nft": executable file not found in $PATH

We should include nft in the k3s-root bundle, so that this works out of the box.

Note: the kube-proxy docs still don't cover the new mode, ref:

• KEP-3866 kube-proxy nftables to beta kubernetes/enhancements#4663 (comment)