delete pre install downloads

scripts/build_upf.sh

@@ -116,7 +116,7 @@ function clear_all()
    rm -rf $VPPPATH/${BUILD_ROOT}/install-vpp_debug-native;
    rm -rf $VPPPATH/${BUILD_ROOT}/*.deb
    rm -rf $VPPPATH/${BUILD_ROOT}/vpp_20.05-rc0~582*
-   
+   rm -rf $VPPPATH/build/external/downloads/*
 }
 
 function help()