Set fail_ci_if_error flag to true

In a recent PR, we got reports from CodeCov about several lines not
being covered. After further inspection, I found an error message in the
coverage file upload saying we hit a rate limit.
This will make it more likely we spot such issues in the future.

The PR: doctrine#6370

.github/workflows/continuous-integration.yml

@@ -658,3 +658,4 @@ jobs:
         uses: "codecov/codecov-action@v3"
         with:
           directory: reports
+          fail_ci_if_error: true