Merge with default KUBECONFIG if no other path is specified

[axisofentropy]

Other Kubernetes configuration actions, including https://github.com/helm/kind-action and the k3s actions on the GHA marketplace, do write the resulting kubeconfig file to the default location .kube/config. Our action should mimic this behavior by default if users do not specify the kubeconfig input.

I think our CLI does this by default now? Check with @zipofar. Maybe we can just drop the --kubeconfig argument from uffizzi cluster create?

The goal is so workflows can follow our action with kubectl or helm without specifying --kubeconfig, as seen in documentation for other Kubernetes configuration actions.

[axisofentropy]

Our action should also set the KUBECONFIG environment variable, either using export or using some GitHub Action facility.

[zipofar]

When we create cluster we can do not define kubeconfig flag. CLI gets path from the env KUBECONFIG or use default path