Function build_action 

pub async fn build_action(
    input: BuildActionInput<'_>,
) -> Result<Option<(Action, Digest)>>

Build the Action envelope for a task and compute its digest.

Returns Ok(None) when the task is not eligible for caching.

Errors

Propagates failures from task command resolution and canonical encoding.

Input hashing failures degrade to Ok(None) so cache eligibility never changes whether the task itself is runnable.