During job run we may fail to resolve actions download info, and this
stack is fully controlled by GitHub actions so it should be counted as
infrastructure failure instead of user failure.
This PR changes GITHUB_ACTION to use the step ContextName, instead of refname. The behavior is behind a feature flag. Refname is an otherwise deprecated property.
Primary motivation: For composite actions, we need a distinct GITHUB_ACTION for each nested step. This PR adds code to generate a default context name for nested steps.
For nested steps, GITHUB_ACTION will be set to "{ScopeName}.{ContextName}" to ensure no collisions.
A corresponding change will be made on the server so context name is never empty. Generated context names will start with "__".
A follow-up PR is required to avoid tracking "step" context values (outputs/conclusion/result) for generated context names. Waiting on telemetry from the server to confirm it's safe to assume leading "__" is a generate context name.
