New metric: failed opensafely "runnable" actions

[sebbacon]

I would like to track the number of action failures for each opensafely repository, so that I can answer questions like:

• Does a large proportion of failing actions correlate with other metrics we care about, like time to complete a project, output checking workload, and system resource usage?
• Is "caring about github actions" primarily a property of an individual or an institution?
• Do low-effort interventions like "tell a copilot that a project's actions are failing" result in changes in behaviour?

I note that it is not possible to query this in any sensible way with GraphQL. Instead we would need to use the REST API.