Skip Docker Buildx setup for TiDB tests

TiDB tests don't need Buildx since testcontainers handles Docker directly.
This prevents unnecessary MySQL image pulls during Buildx initialization.

Author: zph

.github/workflows/main.yml

@@ -167,6 +167,7 @@ jobs:
         run: chmod +x bin/terraform
 
       - name: Set up Docker Buildx
+        if: matrix.db_type != 'tidb'
         uses: docker/setup-buildx-action@v3
 
       - name: Pre-pull Docker images for caching