e2e-gcloud-build to pull latest image instead of a hardcoded version

[leon-gg]

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[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: leon-gg

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[google-oss-prow]

@leon-gg: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ESPv2-e2e-gcloud-build-image	b8fafc5	link	true	/test ESPv2-e2e-gcloud-build-image
ESPv2-presubmit-tsan	b8fafc5	link	true	/test ESPv2-presubmit-tsan
ESPv2-cloud-run-e2e-cloud-function-http-bookstore	b8fafc5	link	true	/test ESPv2-cloud-run-e2e-cloud-function-http-bookstore
ESPv2-cloud-run-e2e-app-engine-http-bookstore	b8fafc5	link	true	/test ESPv2-cloud-run-e2e-app-engine-http-bookstore

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