Pin gcovr Version Until Upstream Issues Are Solved

Version 5.1 of gcovr introduced function coverage, which currently fails since we use the same function name for functions with different footprints for the dynamic library loading symbol names (src/core/module/dynamic_module_impl.cpp).

A very nice analysis of the issue can be found in the related gcovr issue.

Until there is a solution from upstream to this kind of situation (likely a flag to disable the line number assertion check), we have ti pin gcovr to the previous version for our CI to work.