// getGCEUser determines if we're running packer on a GCE, and if we are, gets the associated service account email for subsequent use with OSLogin.
// There are cases where we are running on a GCE, but the GCP metadata server isn't accessible. GitLab docker-engine runners are an edge case example of this.
// It makes little sense to run packer on GCP in this way, however, we defensively timeout in those cases, rather than abort.