When using multi-config CMake generators, executable paths include
per-config subdirectories, which require special handling in tests.
Using dedicated environment variables to specify executable paths
works well in such scenarios. However, the `util_test_runner` test
sets these variables for the `util/test_runner.py` script
unconditionally, which diverges from the approach used when running
`functional/test_runner.py`.
This change makes the usage of the aforementioned environment variables
uniform.
