setup.py --qtpaths or --qmake do not add <qtdir>/bin to PATH

setup.py presently doesn't build properly if your <qtdir>/bin directory is not in the PATH, as during the build of shiboken, Qt6Core.dll is accessed, and setup.py does not add <qtdir>/bin to the path.

If the user specifies {{ --qmake=<qtdir>/bin/qmake.exe}} or --qtpath=<qtdir>/bin/qtpaths.exe to setup.py, those options should be used by setup.py to set the PATH to the <qtdir/bin> directory.

Presently shiboken6 is built if you add <qtdir>/bin to the PATH, but fails the build if <qtdir>/bin is not in the path.