build: have Kokoro grab service account credentials from secret that will be rotated for system tests (#721)

* build: have Kokoro grab service account credentials from secret that will be rotated for system tests

Source-Link: googleapis/synthtool@abbc97d
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest@sha256:fe04ae044dadf5ad88d979dbcc85e0e99372fb5d6316790341e6aca5e4e3fbc8

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* add secret manager keys

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* change secret

* retry automl

* config

* update config

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: sofisl <55454395+sofisl@users.noreply.github.com>
Co-authored-by: Sofia Leon <sofialeon@google.com>

Author: gcf-owl-bot[bot]