Fixed the issue that the generated vrt.xml is empty when writing out COG (#3545)

Author: dingsl-giser

store/src/main/scala/geotrellis/store/cog/vrt/VRT.scala

@@ -172,6 +172,7 @@ object VRT {
       doctype = null
     )
 
+    writer.flush()
     baos
   }