move Windows nightly testing from Jenkins to AppVeyor or Travis-CI or GitHub Actions

[SethTisue]

@lrytz already did the heavy lifting by setting up Windows publishing on AppVeyor, doing this extra bit shouldn't be too hard

once this is done the old jobs on Jenkins can be disabled

[SethTisue]

as per #506, at the time we do this move, we should strongly consider running sbt with -info (the default) instead of -warn. not clear why the old job ever used -warn in the first place

[SethTisue]

this could even be part of PR validation. not sure if it's worth it

also Travis-CI recently rolled out Windows support, so we might not need to bother with AppVeyor. whether there's something preferable about AppVeyor, I have no idea. splitting the workload between multiple competing free services might mean more parallelism

[SethTisue]

as a way to get a little experience before maybe tackling this (though nobody should wait to tackle it themselves because they think I'm going to do it), I did SethTisue/Project-Euler#12

[SethTisue]

Dotty just adopted AppVeyor for this: scala/scala3#9647

[SethTisue]

Dotty is switching from AppVeyor to GitHub Actions: scala/scala3#9934

[SethTisue]

consolidating at #751, since the Windows question is all mixed up with broader questions