fix: rotate certs cp_certs on 20.04 (#5055)

The rotate certs command was failing on 20.04 environments, output:
+ cp -p /etc/kubernetes/rotate-certs/certs/kubeconfig /root/.kube/config
cp: cannot create regular file '/root/.kube/config': No such file or directory

The fix specifies the user instead of using ~, works on both 20.04 and 18.04

parts/k8s/rotate-certs.sh

@@ -30,7 +30,7 @@ cp_certs() {
   cp -p ${NEW_CERTS_DIR}/ca.* /etc/kubernetes/certs/
   cp -p ${NEW_CERTS_DIR}/client.* /etc/kubernetes/certs/
   cp -p ${NEW_CERTS_DIR}/apiserver.* /etc/kubernetes/certs/
-  cp -p ${NEW_CERTS_DIR}/kubeconfig ~/.kube/config
+  cp -p ${NEW_CERTS_DIR}/kubeconfig /home/$(logname)/.kube/config
 
   rm -f /var/lib/kubelet/pki/kubelet-client-current.pem
 }