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.
