The `import/copybara` status check should be required for merging AND expose its output

Currently, the `import/copybara` status check is **not** a required status check, and its output is hidden, making it difficult/impossible for external contributors to debug any failure with it.

I propose that the status check should:

* Be selected as a "required status check" in the Branch Protection settings

* Expose its output such that external contributors can see error messages and derive _why_ it is failing.