add kubectl to PATH

[davidkarlsen]

I think it would be handy to add the tool to PATH like in https://github.com/evryfs/cloudctl-action/blob/master/src/main.ts#L30

[joesteffee]

+1. This is causing the utility to not be found on self-hosted runners for github actions.

[bkalcho]

still experiencing path not found for kubectl with v2.1:

| kubectl apply -n kube-system --kubeconfig ./kubeconfig -f -': exit
│ status 127. Output: /bin/bash: line 23: kubectl: command not found