build: perform publish using Node 12 (#668)

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Source-Link: googleapis/synthtool@5747555

packages/google-cloud-dialogflow/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-dialogflow.git",
-        "sha": "dfad9fa750bf444fc5536f56d1a841536c4cee0c"
+        "sha": "7af5e538bb8c442891af0ef5e85d84c5cefdbf5f"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "bd0deaa1113b588d70449535ab9cbf0f2bd0e72f"
+        "sha": "5747555f7620113d9a2078a48f4c047a99d31b3e"
       }
     }
   ],