Fix step caching

Created by: eseliger

Src side change for https://github.com/sourcegraph/sourcegraph/pull/37240

This fixes two things, that I found today:

• It changes Stdout and Stderr on the StepResult struct to string, it is part of the marshalled cache result and *bytes.Buffer cannot be marshalled into JSON. This caused stdout/stderr to always be empty, thus breaking executions where ${{ previous_step.stdout }} is used.
• It fixes a misconception of the PreviousStepResult. When running the actual execution, it will eventually become the result of the previous step, but from the perspective of that particular cache result, it has to be the result from THAT step execution. Otherwise caching would only work from step 2 on and skip 1 step still.

Test plan

Test case and manual verification.