Include uv configuration in the reviewer workflow checkout (astral-sh#29620)

Reviewer selection fails before requesting a review because its sparse
checkout omits `pyproject.toml`. The script lockfile records the
inherited `exclude-newer` setting, so `UV_LOCKED=1` rejects it when that
configuration is absent. Include `pyproject.toml` in the checkout so the
script can use its committed lockfile.

Co-authored-by: zaniebot <242828183+zaniebot@users.noreply.github.com>

Author: zanieb

.github/workflows/pull-request-reviewers.yml

@@ -37,6 +37,7 @@ jobs:
         with:
           ref: ${{ github.sha }}
           sparse-checkout: |
+            /pyproject.toml
             scripts
             .github/pr-reviewer-pools.toml
           sparse-checkout-cone-mode: false