enh(ci): make the code coverage percentage more precise

Description

The precision of the code coverage as outputted by gcovr is of one digit after the decimal point, but since it also outputs the number of covered lines and the number of total lines, we can calculate this ourselves to achieve a precision of two digits after the decimal point.

Closes #842 (closed)

How to test it?

The precision of the code coverage outputted by the CI should be two decimal after the decimal point. The result should be coherent with the actual code coverage (for example, compare with the HTML report and check if it reports the same value).

Additional tests/tasks to run

For now, bc is installed in the CI script, it should be preinstalled in the docker image.