x/build/internal/relui: allow scheduled workflows to optionally fail fast

[toothrot]

Currently, in relui, a scheduled workflow may fail and send a notification for user intervention. For some workflows, we'd like to be able to have them outright fail and be superseded by the next workflow run.

With the change in https://go.dev/cl/444695, it probably makes the most sense for the WorkflowStalled handler to try to find if a Schedule is configured to fail-fast.