CI: Include coverage data of code which isn't executed by the test suite.

See comment in !151 (merged). Using the "--initial" option of lcov we collect the coverage of all compiled files and merge them later into the final report. This way we can see which files are built but never executed by the test suite.

Because the --initial switch also collects files in the ccache directory we have to point it to the build directory instead, which in turn breaks --no-external. Instead of using --no-external in the collection step, filter out any files not in the source tree in the final coverage job through a path filter.