chore(nodejs): use cloud-rad publication process (#1112) (#500)

VERSION is used in @google-cloud/cloud-rad to publish ref docs for
a particular version. Pass VERSION in via Stubby or Fusion.
Source-Link: googleapis/synthtool@740366b
Post-Processor: gcr.io/repo-automation-bots/owlbot-nodejs:latest@sha256:bbdd52de226c00df3356cdf25460397b429ab49552becca645adbc412f6a4ed5

packages/google-cloud-asset/.trampolinerc

@@ -20,7 +20,8 @@ required_envvars+=(
 
 # Add env vars which are passed down into the container here.
 pass_down_envvars+=(
-    "AUTORELEASE_PR"
+    "AUTORELEASE_PR",
+    "VERSION"
 )
 
 # Prevent unintentional override on the default image.