Use system's PATH value directly

Awesome, thanks! @RobPasMue -- with respect to the changes to `PATH`, I start to wonder if what might be better than trying to undo the addition of the `_internal` directory would either be to:

a) attempt to directly read and use the registry key that Windows would use to set the environment of a newly-started command prompt for the `PATH` specifically -- i.e. what is set in the Control Panel for that environment variable
b) more generally look into ways to launch processes from Python such that they don't inherit the environment from the Python interpreter process but instead emulate the behavior of them being launched by the user from the Windows shell (i.e. `explorer.exe`; the initial environment is based on the Control Panel settings for environment variables). But that would be a separate PR / issue

_Originally posted by @phx-mkoninckx in https://github.com/ansys/python-installer-qt-gui/pull/377#pullrequestreview-2421817094_