evals: drop SUPERPOWERS_ROOT from codex/gemini required_env

These backends only read SUPERPOWERS_ROOT via engine.py/setup.py's
os.environ access, which the new cli.py default helper supplies
automatically. claude*.yaml keep SUPERPOWERS_ROOT in required_env
because they interpolate ${SUPERPOWERS_ROOT} into --plugin-dir args.

evals/backends/codex.yaml

@@ -4,7 +4,6 @@ args:
   - "--dangerously-bypass-approvals-and-sandbox"
 required_env:
   - OPENAI_API_KEY
-  - SUPERPOWERS_ROOT
 hooks:
   pre_run:
     - symlink_superpowers

evals/backends/gemini.yaml

@@ -4,8 +4,7 @@ args:
   - "--yolo"
   - "-m"
   - "gemini-2.5-flash"
-required_env:
-  - SUPERPOWERS_ROOT
+required_env: []
 hooks:
   pre_run:
     - link_gemini_extension