build: have Kokoro grab service account credentials from secret that will be rotated (#1082)

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

Source-Link: https://togithub.com/googleapis/synthtool/commit/4a0230eb8dc497f36fd3839e6144982131f30a9d
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest@sha256:f59941869d508c6825deeffce180579545fd528f359f549a80a18ec0458d7094

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: sha256:5b05f26103855c3a15433141389c478d1d3fe088fb5d4e3217c4793f6b3f245e
+  digest: sha256:f59941869d508c6825deeffce180579545fd528f359f549a80a18ec0458d7094