k8s: fix missing kubeconfig check from endpoint

[crazy-max]

Reported by @jer-k on Slack: https://dockercommunity.slack.com/archives/C7S7A40MP/p1689292389773349

It appears that #1430 (cc: @crazy-max)) removed the commands/util.go file. That file was the only place that called ctxkube.ConfigFromEndpoint:

buildx/driver/kubernetes/context/load.go

Line 168 in 6942118

func ConfigFromEndpoint(endpointName string, s store.Reader) (clientcmd.ClientConfig, error) {

From what I can tell, this makes it so any buildx instances that are using the Kubernetes driver require either

1. KUBECONFIG=<path_to_config> to be prepended to any docker buildx commands
2. KUBECONFIG is exported as an environment variable in your shell
3. .kube/config to point to the cluster

Previously though ctxkube.ConfigFromEndpoint would extract the kubeconfig path from the buildx/instances/<name> file. Was this meant to be removed? It isn't a huge deal as I can resolve my issue with one of the above three approaches. I'm more so just curious 🙂

It has been an oversight during this refactoring.