Buildkite - extract repository slug

[vio]

env-ci extracts the slug using ``${env.BUILDKITE_ORGANIZATION_SLUG}/${env.BUILDKITE_PROJECT_SLUG}`, but on buildkit:

• BUILDKITE_ORGANIZATION_SLUG is the buildkite organization and might differ from repo organization
• BUILDKITE_PROJECT_SLUG (obsolete) is the buildkite project slug (buildkite organization/buildkite project name)

ref: https://buildkite.com/docs/pipelines/environment-variables

relative-ci:agent env-ci environment variables {
  isCi: true,
  service: 'buildkite',
  slug: 'rltv/rltv/example',
  branch: 'update-agent',
  pr: undefined,
  build: '9',
  buildUrl: 'https://buildkite.com/rltv/example/builds/9',
  commit: '1b6a3c967ada2e7f85f9ea12d25fd3e4523edd5e',
  commitMessage: undefined