Fix platform not supported error in build.py (#4102)

scripts/build.py

@@ -694,7 +694,7 @@ def main(args):
     orig_branch = get_current_branch()
 
     if args.platform not in supported_builds and args.platform != 'all':
-        logging.error("Invalid build platform: {}".format(target_platform))
+        logging.error("Invalid build platform: {}".format(args.platform))
         return 1
 
     build_output = {}