fix deploy: install git in build-package step

The uv slim image lacks git, which is needed for commit-count
based version stamping.

.woodpecker/deploy.yml

@@ -12,6 +12,7 @@ steps:
   - name: build-package
     image: ghcr.io/astral-sh/uv:python3.13-bookworm-slim
     commands:
+      - apt-get update -qq && apt-get install -y -qq git > /dev/null
       # Stamp a unique version: base version + .dev{commit_count}
       - COMMIT_NUM=$(git rev-list --count HEAD)
       - BASE=$(grep '^version' pyproject.toml | head -1 | sed 's/.*"\(.*\)"/\1/')