onboard-spoke.sh: replace raise SystemExit with sys.exit(0) in union duplicate check

hub/onboard-spoke.sh

@@ -182,7 +182,7 @@ for line in lines:
         in_section = False
     if in_section and line.startswith("upstreams =") and prefix in line:
         print("yes")
-        raise SystemExit
+        sys.exit(0)
 print("no")
 PYEOF
 )