temporary fix for lcov on debian 13 (trixie)

freezing the lcov version to 1.16 which is the latest installed automatically on debian 12 (bookworm)

symptom: https://gitlab.cern.ch/mro/third-party/grbl/grbl-gpiod/-/jobs/62145521

issue: https://github.com/linux-test-project/lcov/issues/398