Generate Gateway API CRD only when kubectl exists (#215)
Generate Gateway API CRD only when kubectl exists (#215)

As current script generates the Gateway API CRD by `kubectl kustomize`,
it fails when kubectl does not exist.
E.g. "Update Deps and Codegen" in knative-sandbox/knobots misses the command as
https://github.com/knative-sandbox/knobots/runs/4408339616?check_suite_focus=true

So this patch changes to generate the CRD only when kubectl command exists.
knative-sandbox/net-gateway-api,knative-sandbox/net-gateway-api