scripts: Only download kernel binaries if changed

Only download the kernel binaries from the github release if the remote
file is newer (avoids multiple copies accumulating in the download
directory.)

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

scripts/test-util.sh

@@ -143,7 +143,7 @@ download_linux() {
     fi
     pushd "$WORKLOADS_DIR" || exit
     for url in $KERNEL_URLS; do
-        wget --quiet "$url" || exit 1
+        wget -N --quiet "$url" || exit 1
     done
 
     popd || exit