buildkite: persist build org so /logs honours workflow override #41

A workflow can override the spindle's default Buildkite organisation via tack.buildkite.org, but BuildkiteBuildRef didn't carry the org field. Spawn used the override for CreateBuild and then dropped it, so Logs always recomputed org := p.defaultOrg and any cross-org workflow's /logs request 404'd against the wrong organisation.

Extend TestBuildkiteSpawnWorkflowConfig to assert the org survives the round-trip via LookupBuildkiteBuildByTuple.