Build release based on relocatable python

[pkienzle]

Rather than reimplementing the python CLI for SasView (#2280) we can drop sasview.exe and use relocatable python to run the app. This is what @bmaranville did for refl1d:

https://github.com/reflectometry/refl1d/blob/master/.github/workflows/test.yml
https://github.com/reflectometry/refl1d/blob/master/extra/build_win_installer.ps1

You can even get pip to work in the installed distributions with some small changes (see pypa/pip#9742, pypa/pip#4368).