Hello Tom,
Thank you for your suggestion. Currently Cloud Build supports three types of machine which is also mentioned in this article [1]. However I would recommend to open a feature request by using this issue tracker link [2] so we can understand your expectations more clearly. I have provided the appropriate component in the link for this request. This is worth mentioning that there are no guarantees or ETAs to implement the feature requests. However we really appreciate new ideas from customers.
[1] https://cloud.google.com/cloud-build/docs/build-config#options
[2] https://buganizer.corp.google.com/issues/new?component=190802&template=1162743