[sdk-platform-java] The Hermetic Build integration tests should be run in GCB

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](https://github.com/googleapis/sdk-platform-java/blob/01e742de49e151efbd903808a859f595f99bc8de/.github/workflows/verify_library_generation.yaml#L41-L63). 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.