ci: support release in `dry-run` mode

[v1v]

What

Enable the release workflow running on Buildkite and being called using the GitHub action workflow. It will not push anything but either run the dry-run mode or gradle assemble.

Follow-ups

• Support for the release gradle command.
• Support for the post-release steps, similarly done in https://github.com/elastic/apm-agent-android/blob/main/build-tools/src/main/java/co/elastic/apm/compile/tools/publishing/tasks/PostDeployTask.java