perf(handlers): improve performance of repository existence check for GCS

mentioned in commit e4f20a74

mentioned in merge request gitlab-org/build/CNG!812 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1323 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2250 (merged)

mentioned in merge request omnibus-gitlab!5709 (merged)

mentioned in merge request gitlab-development-kit!2385 (merged)