Fix utils/build-script unbound variable.

Fix error introduced in #29997.
Todo: minimize utils/build-script diff with master branch.

Author: dan-zheng

utils/build-script

@@ -1160,6 +1160,7 @@ def main_preset():
     if not args.preset:
         fatal_error("missing --preset option")
 
+    args.preset_substitutions = {}
     for arg in args.preset_substitutions_raw:
         name, value = arg.split("=", 1)
         args.preset_substitutions[name] = value