Fix ci test coverage report
Gitlab s CI syntax for reports has changed, especially cobertura/coverage reports.
Additionally it seems files outside the build directory are discarded after script execution and before artifact processing begins.
See gitlab-org/gitaly!4586, gitlab-org/gitaly!4594
Fixed by
- Revert change into /app directory, remain inside build directory to preserve artifacts
- Adjust report artifact instructions for coverage job see docs