github-action: run buildkite action with GH secrets

[v1v]

Use GitHub secrets to run the Buildkite release/snapshot

No more Vault secrets access.

There is a pending task to replace the GitHub token user.

Tests

I created a feature branch called test/buildkite-token and triggered a dry-run snapshot:

• https://github.com/elastic/ecs-logging-java/actions/runs/9365973088

[SylvainJuge]

FYI I've renamed the release.yml file in #260, so if it's merged before this one you will likely have to double-check that your changes are included in the release-step-3.yml.