fix: prevent PR repo's .python-version from overriding setup-python

[Shimada666]

Problem

When the PR review action runs uv run inside pr-repo/, uv discovers and respects the reviewed repository's .python-version file. If that file specifies Python < 3.12 (e.g. 3.11), uv downloads that version instead of using the 3.12 installed by setup-python, causing openhands-sdk installation to fail:

Downloading cpython-3.11.15-linux-x86_64-gnu (download) (29.9MiB)
× No solution found when resolving `--with` dependencies:
╰─▶ Because the current Python version (3.11.15) does not satisfy Python>=3.12
    and all versions of openhands-sdk depend on Python>=3.12,
    we can conclude that all versions of openhands-sdk cannot be used.

Even though --no-project is passed to ignore pyproject.toml, uv still reads .python-version for Python version discovery.

Fix

Set UV_PYTHON_PREFERENCE: only-system on the "Run PR review" step so uv always uses the system Python installed by setup-python and ignores any .python-version file in the reviewed repository.

Test plan

• Verified the fix on our repo (MizzenAI/Personality) which has .python-version: 3.11 and was consistently failing with this error.

[all-hands-bot]

🟢 Good taste - Elegant, minimal fix that solves a real problem.

[RISK ASSESSMENT]
⚠️ Risk Assessment: 🟢 LOW

One-line environment variable change with clear, predictable behavior. Forces uv to use the system Python installed by setup-python, eliminating the failure mode when reviewed repos have .python-version files specifying Python < 3.12. No breaking changes, no security concerns.

VERDICT: ✅ Worth merging

KEY INSIGHT: Fixing implicit dependencies (uv reading .python-version from the reviewed repo) with explicit configuration (UV_PYTHON_PREFERENCE: only-system) is exactly the right approach - it makes behavior predictable and eliminates special cases.