Reintroduced the windows installer link on the download page - Added a buildbot badge

[Friedrich Beckmann]

I have reintroduced the windows installer link to the nightly windows installer 
builds on the buildbot on the gnu pspp website.

See: https://www.gnu.org/software/pspp/get.html

I also added the nice buildbot badge! I have a direct link to the nightly build 
of today because that one at least did not crash when I ran the installer on a 
windows 11 machine. It would be better to have a windows installer based on a 
released version but I guess the windows build does not work in the last 1.4.1 
release.

Cheers

Fritz