contrib: fix `test_deterministic_coverage.sh` script for out-of-tree builds

The determinstic line coverage checking script test_deterministic_coverage.sh was seemingly forgotten to be adapted in the course of switching from autotools to CMake (only the error messages containing build instructions were updated in a9964c04 / #30875). Since gcovr needs to access both the source and build files, the path to the latter has to be passed explicitly as they are not in the same anymore (see e.g. https://github.com/gcovr/gcovr/issues/340).

Can be tested by:

$ cmake -B build -DCMAKE_BUILD_TYPE=Coverage
$ cmake --build build
$ ./contrib/devtools/test_deterministic_coverage.sh

Alternatively, if the script is not needed anymore (apparently no-one else tried to execute it for the last three months?), it could also be deleted.