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

[gcf-owl-bot[bot]]

Source-Link: https://github.com/googleapis/synthtool/commit/abbc97db69a57dcb991ba97ef503305b701ffb3a Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest@sha256:fe04ae044dadf5ad88d979dbcc85e0e99372fb5d6316790341e6aca5e4e3fbc8