build: migrate to secret manager (#584)

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Source-Link: googleapis/synthtool@1c92077

Author: yoshi-automation

core/common/.kokoro/populate-secrets.sh

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-common.git",
-        "sha": "0b92ab0aff042de3520f174d4877ebde0b451d8b"
+        "sha": "8d12007c5b43b8b6a679b0a925c05cb665d850b7"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "be74d3e532faa47eb59f1a0eaebde0860d1d8ab4"
+        "sha": "1c92077459db3dc50741e878f98b08c6261181e0"
       }
     }
   ]