buildkite: persist build org so /logs honours workflow override
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.
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