Fixed stacking prompt instructions (#5520)

fixed issue with prompt instruction stacking

prompt instructions were stacking making the agent eventually return an error because the prompt was full of repeating instructions

autogpts/autogpt/autogpt/agents/agent.py

@@ -108,10 +108,12 @@ class Agent(
     def build_prompt(
         self,
         *args,
-        extra_messages: list[ChatMessage] = [],
+        extra_messages: [list[ChatMessage]] = None,
         include_os_info: Optional[bool] = None,
         **kwargs,
     ) -> ChatPrompt:
+        if extra_messages is None: 
+            extra_messages = []
         # Clock
         extra_messages.append(
             ChatMessage.system(f"The current time and date is {time.strftime('%c')}"),