build: perform publish using Node 12 (#385)

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Source-Link: googleapis/synthtool@5747555

packages/google-cloud-iot/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-iot.git",
-        "sha": "88666253443952c2b4445573e92df180e8691498"
+        "sha": "717b0b905329c48bb3a862ff73984020db71ebc4"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "bd0deaa1113b588d70449535ab9cbf0f2bd0e72f"
+        "sha": "5747555f7620113d9a2078a48f4c047a99d31b3e"
       }
     }
   ],