fix(gcp): reduce scope of GCP App Default Creds script to template

mentioned in commit a642ffe5

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!74

mentioned in merge request to-be-continuous/samples/maven-on-kubernetes!141 (merged)

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!79 (closed)

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!81