build(node): add KOKORO_BUILD_ARTIFACTS_SUBDIR to env (#220)

This PR was generated using Autosynth. 🌈

Synth log will be available here:
https://source.cloud.google.com/results/invocations/933ad2fd-a72d-472d-91bb-4b474e01ed77/targets

- [ ] To automatically regenerate this PR, check this box.

Source-Link: googleapis/synthtool@ba9918c

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-recaptcha-enterprise.git",
-        "sha": "f0edc8422d39eba895bd533177758df80f8922a6"
+        "sha": "abf59a948ff90aead2b5e43d85edcefab92ec534"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "89c849ba5013e45e8fb688b138f33c2ec6083dc5"
+        "sha": "ba9918cd22874245b55734f57470c719b577e591"
       }
     }
   ],