build(node): add KOKORO_BUILD_ARTIFACTS_SUBDIR to env

[yoshi-automation]

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[codecov]

Codecov Report

Merging #1094 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1094   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files          21       21           
  Lines        4145     4145           
  Branches      461      461           
=======================================
  Hits         3798     3798           
  Misses        347      347           

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