Calculating default buildToolsVersion

Question

According to the Android SDK Release Notes you no longer need to define the buildToolsVersion property in the project's build.gradle, as a default version is used

If you're using Android plugin for Gradle 3.0.0 or higher, your project automatically uses a default version of the build tools that the plugin specifies.

However, I am trying to configure our continuous integration server to sign the compiled application and I can't figure out which version of build tools to use.

If the build.gradle defines a buildToolsVersion then this can be extracted, but how would I find the "default" version pragmatically if none is defined?

All the documentation says to use the "default" or "recommended" version, but never defines how to find this