Part 1/3 of #62317, split for easy revert. Guards post-task steps against no-op cases so post_error_fails_task (part 3) doesn't fail legitimately-empty tasks.

• Guard subprocess.exec post steps that run scripts absent when there's no source checkout
• Mark s3.put/attach.artifacts optional: true where output isn't always produced
• Tolerate missing powercycle artifacts in tar via --ignore-failed-read
• Fix hang analyzer task generation when no Evergreen API credentials are configured
• Restore the patched tree on exit in feature_flag_tags_check.sh