build(node): add KOKORO_BUILD_ARTIFACTS_SUBDIR to env (#668)

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Source-Link: googleapis/synthtool@ba9918c

packages/google-cloud-node/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-speech.git",
-        "sha": "f43a11ae265c30cc5767e75c53997070380537ca"
+        "sha": "c466c1251b43fe9437b9cd0e04b0875d86385178"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "89c849ba5013e45e8fb688b138f33c2ec6083dc5"
+        "sha": "ba9918cd22874245b55734f57470c719b577e591"
       }
     }
   ],