Switch to sys.base_prefix for better Lib path detection

[Linus-XZX]

This changes the approach to finding the Lib directory, from under parent dir of sys.executable to under sys.base_prefix.

The current approach fails when the Python interpreter invoked is from a virtual environment, since in that case the executable becomes \path\to\venv\Scripts\python.exe where the prefix Lib is \path\to\venv\Lib.

Changing src/proxy/build_pythoncore.py line 7 to use .parent.parent will make the assert go through, but then result in an empty pythoncore.zip, since the real libraries to include in the archive are under the base prefix rather than the venv's Lib.

This change should not hurt either if the interpreter is not in a venv, since in that case sys.base_prefix is just the same as the parent of sys.executable.

Tested with the base environment and venv, on both py managed and uv managed Python installations.