Update CI workflow to specify Tsinghua mirror for Python package installation

.gitea/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
 
       - name: 安装后端依赖
         working-directory: back
-        run: pip3 install --break-system-packages -r requirements.txt
+        run: pip3 install --break-system-packages -i https://pypi.tuna.tsinghua.edu.cn/simple -r requirements.txt
 
       - name: 构建 Docker 镜像
         run: docker-compose build