There are no instructions to run the hermetic build ITs locally

[diegomarquezp]

I found some potential improvements for https://github.com/googleapis/sdk-platform-java/blob/main/hermetic_build/DEVELOPMENT.md#run-the-integration-tests. Mainly, to explain how to run the integration test locally.

For example, #3472 simplified the IT setup and moved it to Cloud Build in order to leverage Airlock and parallelize the different setup stages of the IT. The configuration for such IT also introduced changes in the way it's run.

With such guide, the commits after 6b5c220 as part of #3723 would have probably been less.

After inspecting the IT's .cloudbuild file and related files, I understood that:

• The generation config at googleapis/google-cloud-java@f20c311 has no effect. Instead, the golden configuration in sdk-platform-java is used. Although I was involved in this codebase, it's a fact I forgot and had me wondering why the generation produces a mismatch with the golden branch :)
• The Dockerfile for the integration tests is not the traditional one, but the Airlock version.
• The generator is separately downloaded and locked to a specific version that overrides the one compiled in the image building job.