Add status badges for workflows to README

[ianlewis]

I'd like to have a section in the top level README that showed status badges for workflows so you can tell at a glance which are succeeding and failing.

Unfortunately many workflows run with a shim which skips running for the wrong triggers, and jobs that push or create releases when run on a schedule or via workflow_dispatch. We would need to break up the shim job and main workflow into separate workflow files for this to work properly.

[ianlewis]

I would like to use the build-in GHA badges since they best reflect when the workflow is failing. But we need to make sure to skip all jobs in the workflow if the workflow is meant to skip the event. This means our shim job steps need to be reevaluated. They currently run and succeed making the whole workflow look as if it has succeeded.

We also need to make sure adversarial workflows have success status on an expected failure.

Related #178