Entrypoint: support updating credentials

[tylerjwatson]

The entrypoint.sh script will only attempt to apply the contents of the KUBE_CONFIG variable when ~/.kube/config doesn't exist, which means if the credentials in the KUBE_CONFIG variable changes the image is stuck with the old ones, rendering the action useless when organization secrets change.

This patch removes the requirement for environment variables to be applied only when ~/.kube/config doesn't exist.

The environment variables are passed every run, they should be regarded every run.