The Hermetic Build integration tests should be run in GCB

[diegomarquezp]

Currently, our Docker image is built and pushed after a PR is merged. This setup doesn't catch situations where a new build procedure may work differently or simply break in the posterior GCB build.

For example, after merging https://github.com/googleapis/sdk-platform-java/pull/3324, which contained changes to the Dockerfile, we started seeing build errors not captured in our integration tests. This is because the integration test uses the GH actions environment. Therefore, moving our ITs to GCB will replicate the build behavior that occurs after merging the PR and would prevent any build errors from being merged.