ci: point uv build at Tsinghua PyPI mirror via UV_DEFAULT_INDEX

The self-hosted astral-uv runner is in China; pulling build deps
from pypi.org is slow and often times out. Set UV_DEFAULT_INDEX
on the 'Build package' step so uv's build-environment resolution
goes through the Tsinghua mirror.

This complements the [tool.uv.index] url=... block in
pyproject.toml — env wins over the file config when both are
present, so the mirror is used consistently.

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/build.yml

@@ -21,6 +21,11 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Build package
+        env:
+          # Use the Tsinghua PyPI mirror for build-environment resolution;
+          # the self-hosted runner is in China and the default pypi.org is
+          # too slow / unreliable for build dependency install.
+          UV_DEFAULT_INDEX: "https://pypi.tuna.tsinghua.edu.cn/simple/"
         run: uv build
 
       - name: Upload wheel artifacts