wget: no cache is needed.
wget: no cache is needed.

Before this change, as io.Copy reads from source
and write to destination, it caches a copy.

wget simply need to dump contents, caching a copy
while reading is unnecessary, and can cause OOM in
systems with limited ram.

Signed-off-by: David Hu <a59597aa737a9c4d32eaf7b77687719fab10c4c5@google.com>

u-root/u-root,u-root/u-root,u-root/u-root