fix: use kubeadm v1beta4 list format for kubeletExtraArgs

kubeadm v1beta4 expects nodeRegistration.kubeletExtraArgs as a list of name/value args, not a map. Switch hostname-override to the correct structure so init config unmarshals successfully.

nixos/kubeadm/modules/k8s-common.nix

@@ -165,7 +165,8 @@ in
         name: "KUBEADM_NODE_NAME"
         criSocket: unix:///run/containerd/containerd.sock
         kubeletExtraArgs:
-          hostname-override: "KUBEADM_NODE_NAME"
+          - name: hostname-override
+            value: "KUBEADM_NODE_NAME"
       ---
       apiVersion: kubeadm.k8s.io/v1beta4
       kind: ClusterConfiguration