This is an excellent question, and definitely a place where we can improve build times (npm install being one of the steps that take the longest)
While in this document the focus was to try to keep the pipeline as simple as possible. If you wanted to implement npm cache we could use google cloud storage as a sort of "persistant disk" where we could cache package for subsequent builds. We could then add steps to add the packages to the npm cache as part of one of our build steps before npm install.
More info can be found here as to how we woud go about that
cloud.google.com/build/docs/speeding-up-builds