build(node): add KOKORO_BUILD_ARTIFACTS_SUBDIR to env (#495)

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Source-Link: googleapis/synthtool@ba9918c

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-monitoring.git",
-        "sha": "ae747c3b83662c56f35cef9e769c2a93499f0e4d"
+        "sha": "6445e65c0ff84e9cb0dac2c9b7bf172300f4bd2a"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "89c849ba5013e45e8fb688b138f33c2ec6083dc5"
+        "sha": "ba9918cd22874245b55734f57470c719b577e591"
       }
     }
   ],