ci: Get gcovr from pip on all images
Like at TinySPARQL:
Somewhat recently, gcovr bumped the internal format of its JSON output,
this got noticed first at Rawhide, but now it seeped into latest Fedora.
Using the assumption that there are larger compatibility guarantees in
the compiler output, let's use gcovr from pip so all test containers
have a common baseline to extract the coverage information.