interpreter: only calculate build_filename if we're actually going to use it

mesonbuild/interpreter/interpreter.py

@@ -313,8 +313,8 @@ class Interpreter(InterpreterBase, HoldableObject):
         # exist we don't want to add a dependency on it, it's autogenerated
         # from the actual build files, and is just for reference.
         self.build_def_files: mesonlib.OrderedSet[str] = mesonlib.OrderedSet()
-        build_filename = os.path.join(self.subdir, environment.build_filename)
         if not is_translated:
+            build_filename = os.path.join(self.subdir, environment.build_filename)
             self.build_def_files.add(build_filename)
         self.parse_project()
         self._redetect_machines()