scripts: fix the check for GitHub authentication token

When the script is invoked via dev_cli.sh, it always gets AUTH_DOWNLOAD_TOKEN
from the environment. The original test always returns true.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

scripts/test-util.sh

@@ -109,7 +109,7 @@ process_common_args() {
 }
 
 download_hypervisor_fw() {
-    if [ -v AUTH_DOWNLOAD_TOKEN ]; then
+    if [ -n "$AUTH_DOWNLOAD_TOKEN" ]; then
         echo "Using authenticated download from GitHub"
         FW_URL=$(curl --silent https://api.github.com/repos/cloud-hypervisor/rust-hypervisor-firmware/releases/latest \
                  --header "Authorization: Token $AUTH_DOWNLOAD_TOKEN" \