Split building from uploading the Prototype Build

[mokagio]

Update the local automation and how it is called in Buildkite to split building the Prototype Build for Enterprise distribution via Firebase from uploading it to Firebase.

Use Pocket Casts iOS as a reference. In particular:

• https://github.com/Automattic/pocket-casts-ios/blob/6e83124130c4007079592184a43af1f9c234a820/fastlane/Fastfile#L667-L738
• https://github.com/Automattic/pocket-casts-ios/blob/6e83124130c4007079592184a43af1f9c234a820/.buildkite/pipeline.yml#L47-L76

Unlike Pocket Casts iOS, however, prototype builds from PRs on this project do not require user input to begin; they should run on every build triggered from a PR.

See also https://linear.app/a8c/issue/AINFRA-1624/split-prototype-build-build-from-upload-via-dedicated-ci-steps-in

[dangermattic]

Thanks for reporting! 👍

[mokagio]

Created this to see how Copilot will cope with it. But waiting for #16432 to be merged before assigning it.