bug: harness reviewer is failing on codebuild hosted runners.

[Hweinstock]

Problem

Example: https://github.com/aws/agentcore-cli/actions/runs/29121238023/job/86456628695?pr=1739

Run actions/setup-python@v6
Installed versions
  Version 3.12 was not found in the local cache
  Error: The version '3.12' with architecture 'x64' was not found for amzn 2023.
  The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Looks like the new machines don't support the requested version of python.

This is causing failures in CI, introducing noise to review environments.

Definition of Done

• fix the workflow to use a compatible runner OR downgrade the version to something compatible with the runner.