Use built in Environment Variables by default instead of default Substitution variables for Cloud Build
Unlike nearly every automated build system, Google Cloud Build does not set environment variables inside of the running build docker containers automatically with the build context information. Instead it has "Substitution Variables" that a developer has to explicitly pass as environment variables. This makes tools that customize information based on the build environment impossible without special configuration to pass the default substitution variables as environment variables to each build step.
By default the default substitution variables should be available as environment variables in the build containers. Maybe google could provide an option to disable that functionality for extremely sensitive build steps, but if this new functionality isn't the default behavior, it means Cloud Build is not on feature par with most other build systems and tools like "env-ci" will never work with them without special configuration (defeating the purpose of the library).