Python binding is broken: it ignores PYTHON_EXECUTABLE and installs into a wrong prefix

1. When multiple puthon versions are installed ```PYTHON_EXECUTABLE``` is used to set which python to use.

I set PYTHON_EXECUTABLE=/usr/local/bin/python3.8 but 3.10 is still used

2. It installs python binding into the root directory / instead of the python site libs folder.

Reproduce the two reported cases with PYTHON_EXECUTABLE=/usr/local/bin/python3.8: verify which Python version is selected and where the binding is installed. Trace the Python binding configuration and installation-prefix handling, then confirm that the requested interpreter is used and the binding lands in that interpreter's site-libraries folder.

Written by the indexing model from the issue text.