setup-spoke.sh: redirect find_free_port warn output to stderr to prevent contamination of TUNNEL_PORT variable

spoke/setup-spoke.sh

@@ -308,7 +308,7 @@ find_free_port() {
             echo "$port"
             return 0
         fi
-        warn "Port $port is in use, trying next..."
+        echo -e "${YELLOW}[!]${NC} Port $port is in use, trying next..." >&2
     done
     return 1
 }