Update build.func

update ssh_check code

misc/build.func

@@ -67,7 +67,7 @@ arch_check() {
 
 ssh_check() {
   if command -v pveversion >/dev/null 2>&1; then
-    if [ -n "$SSH_CLIENT" ]; then
+    if [ -n "${SSH_CLIENT:+x}" ]; then
       if whiptail --defaultno --title "SSH DETECTED" --yesno "It's suggested to use the Proxmox shell instead of SSH, since SSH can create issues while gathering variables. Would you like to proceed with using SSH?" 10 62; then
         echo "you've been warned"
       else