fix CI: use system Python to avoid network downloads

Set UV_PYTHON_PREFERENCE=only-system so uv uses the Python
bundled in the container image instead of downloading from GitHub.

.woodpecker/ci.yml

@@ -8,6 +8,8 @@ when:
 steps:
   - name: lint
     image: ghcr.io/astral-sh/uv:python3.13-bookworm-slim
+    environment:
+      UV_PYTHON_PREFERENCE: only-system
     commands:
       - uv sync --dev
       - uv pip install -e .
@@ -16,6 +18,8 @@ steps:
 
   - name: test
     image: ghcr.io/astral-sh/uv:python3.13-bookworm-slim
+    environment:
+      UV_PYTHON_PREFERENCE: only-system
     commands:
       - uv sync --dev
       - uv pip install -e .