Fix Failing GitHub Actions

[ ] 👍 Sweep Did Well
[ ] 👎 Sweep Needs Improvement I created this PR to fix the failing GitHub Actions.## Description This PR addresses potential issues causing GitHub Actions to fail. The changes include improvements to the setup process, test environment, linting, and GitHub Actions workflows.

Summary of Changes

Modified cli.py to improve the setup process. Added checks for environment variables and existing dependencies, and improved error logging for GitHub access token validation.
Updated autogpt/autogpt/tests/conftest.py to ensure correct scoping and teardown of fixtures, and added a fixture to mock external API calls.
Created a new GitHub Actions workflow .github/workflows/lint.yml for linting the codebase on every push and pull request.
Updated the main GitHub Actions workflow .github/workflows/main.yml to add caching for dependencies, ensure correct test environment, and add a linting step.

These changes aim to make the GitHub Actions workflows more robust and efficient, and improve error logging for easier debugging.