fixed issue with prompt instruction stacking

[NeonN3mesis]

Background

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

Changes 🏗️

made sure to reset extra_messages in build_prompt

PR Quality Scorecard ✨

• Have you used the PR description template?   +2 pts
• Is your pull request atomic, focusing on a single change?   +5 pts
• Have you linked the GitHub issue(s) that this PR addresses?   +5 pts
• Have you documented your changes clearly and comprehensively?   +5 pts
• Have you changed or added a feature?   -4 pts
  • Have you added/updated corresponding documentation?   +4 pts
  • Have you added/updated corresponding integration tests?   +5 pts
• Have you changed the behavior of AutoGPT?   -5 pts
  • Have you also run agbenchmark to verify that these changes do not regress performance?   +10 pts