fix(KUBECONFIG): select to the given KUBE_CONTEXT if provided

mentioned in commit c7d0fbf4

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!22 (merged)

mentioned in merge request to-be-continuous/samples/python-on-kubernetes!8 (merged)

mentioned in merge request to-be-continuous/samples/php-cnb-on-kubernetes!20 (closed)

mentioned in merge request to-be-continuous/samples/php-on-kubernetes!65 (merged)

mentioned in merge request to-be-continuous/samples/php-cnb-on-kubernetes!23 (merged)