Update build.sh

Author: AliyunWorkbench

build.sh

@@ -1,7 +1,9 @@
 #!/bin/bash
+rm -rf build
+rm -rf public
 # 生成hexo静态文件
 npm install hexo-cli
 npm install
 ./node_modules/hexo-cli/bin/hexo generate
-mkdir build
+hexo generate
 cp -r public build