This feature allows GitLab to visualize test coverage information in the
file diff view of merge requests.

This commit makes the gcov CI job depend on the following chain of jobs:

    gcc:buster:amd64 → unit:gcc:buster:amd64 → system:gcc:buster:amd64

The reason for running the last two jobs above sequentially rather than
in parallel is that both of them create *.gcda files (containing
coverage data) in the same locations.  While some way of merging these
files from different job artifact archives could probably be designed
with the help of additional tools, the simplest thing to do is not to
run unit test and system test jobs in parallel, carrying *.gcda files
over between jobs as gcov knows how to append coverage data to existing
*.gcda files.

Also note that test coverage will not be visualized if any of the jobs
in the above dependency chain fails (because the gcov job will not be
run).