build: have Kokoro grab service account credentials from secret that will be rotated for system tests (#446)

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

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -13,4 +13,4 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest
-  digest: sha256:f59941869d508c6825deeffce180579545fd528f359f549a80a18ec0458d7094
+  digest: sha256:fe04ae044dadf5ad88d979dbcc85e0e99372fb5d6316790341e6aca5e4e3fbc8