This adds a simple fallback Buildkite pipeline configuration which
always fails the pipeline, but correctly reports back the failure
status.
Note that this also requires changes in the Buildkite configuration
that is not in version-control.
Relates to b/66.
Change-Id: I6802a6f76448c3893798a06d514e6ccba0f50dd2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2102
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>