Add checkout step to publish

[leonardehrenfried]

The master branch build failed and I believe it was because of a missing checkout action.

cc @carlomion

[what-the-diff]

PR Summary

• Introduction of New Checkout Step
  The addition of a "Checkout" step has been made to the continuous integration (.github/workflows/ci.yml) file. This ensures that the automated systems always have the latest version of the project files to work with, leading to consistency in testing and deploying.