Simplify caching logic in 'src batch exec'

Created by: mrnugget

This is part of https://github.com/sourcegraph/sourcegraph/issues/26929 and simplifies the caching-related logic in src batch exec to only what's needed: checking for per-step cache results.

Since server-side batch changes use server-side caching for changeset specs we don't need to check for changeset specs.