I've been looking all over for this, and finally discovered a setting in App Store Connect --> Xcode Cloud --> Settings --> Build Number. Not sure if there's a way to set this with an outside process, but you can at least set a starting number and it should auto increment.

App Store Connect --> Xcode Cloud --> Settings --> Build Number. You'll at least be able to set the next build number, and it should auto increment after that. I haven't found a way to pull from CFBundleVersion explicitly yet though.