buildkite: add permissions to observablt-robots-automation

[v1v]

What does this PR do?

Use a machine account which it's member of https://github.com/orgs/elastic/teams/observablt-robots-automation

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation
• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix
• This is a new plugin
  • I have updated CHANGELOG.asciidoc
  • My code follows the style guidelines of this project
  • I have made corresponding changes to the documentation
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
  • I have updated supported-technologies.asciidoc
  • Added an API method or config option? Document in which version this will be introduced
  • Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• This is something else
  • I have updated CHANGELOG.asciidoc

[SylvainJuge]

Is there any reason to not always have BUILD_AND_READ for observablt-robots-automation ?

From what I understand observablt-robots-automation is only used for automation and machine jobs, and I don't think we need to allow managing the jobs themselves, which I would have expected to be a human-only responsibility.

[v1v]

BUILD_AND_READ is now the default behaviour