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

[codecov[bot]]

Codecov Report

Base: 68.87% // Head: 68.87% // No change to project coverage :thumbsup:

Coverage data is based on head (86b962e) compared to base (62d7e4f). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #866 +/- ## ======================================= Coverage 68.87% 68.87% ======================================= Files 7 7 Lines 1253 1253 Branches 58 58 ======================================= Hits 863 863 Misses 389 389 Partials 1 1 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=googleapis). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=googleapis)

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