Fix typo in reset func in productpipeline_list_builder.py (#82777)

`pipelinst_list` -> `pipeline_list`

There are no other uses of `pipelinst_list`, so it's clearly a typo when trying to refer to `pipeline_list`

Author: MaxDesiatov

utils/swift_build_support/swift_build_support/productpipeline_list_builder.py

@@ -91,7 +91,7 @@ def reset(self):
         self.current_count = 0
         self.current_pipeline = None
         self.is_current_pipeline_impl = False
-        self.pipelinst_list = []
+        self.pipeline_list = []
 
     def add_product(self, product_cls, is_enabled):
         """Add a non-impl product to the current pipeline begin constructed"""