Cirrus CI (Windows help wanted)

Hi,

My new favourite CI is Cirrus CI, because it has 4 operating systems,
generous enough quotas to handle 250+ branches in a single account,
and public build/test log URLs. I flipped cfbot.cputube.org (mostly)
over to that and it seems to work well so far -- fingers crossed.
I've also been using it for my own development branches that involve
some systems hacking-heavy work that uses different kernel interfaces
on all 4 of those OSes.

There's one thing I'm stuck on, though: Windows. If anyone wants to
help figure out how to get PostgreSQL to build on Cirrus's Windows,
I'd be very interested. To play with this stuff, you need a public
Github repo, and you need to add Cirrus CI from "Marketplace" (it's
free for public/open source), and then you add a .cirrus.yml file such
as https://github.com/macdice/cfbot/blob/master/cirrus/.cirrus.yml to
the top level of a PostgreSQL branch. When you push, you should see
build results on the Github web UI.

For a similar example that works on Windows on another CI, see
https://github.com/macdice/cfbot/blob/master/appveyor/appveyor.yml
(note that it also references a couple of other files; it'd be nice to
be able to do that stuff without the need for separate files, possibly
by using Power Shell). That's what cfbot is using for Windows for
now, which works really well, but it'd be nice to have more
options/choices. For another example of Windows builds working on
another CI, see the Github Actions patch I posted earlier when I was
considering that for cfbot[1]. I think what's different is that those
other CIs have images with MSVC on them, but Cirrus wants you to
figure out how to install the right toolchain yourself (and then, as a
next step after it's actually working, it also provides a way to
define what you want in a way that captures the resulting image using
Docker voodoo, so that you get fast startup times). Or something.

[1] https://www.postgresql.org/message-id/flat/CA%2BhUKG%2By_SHVQcU3CPokmJxuHp1niebCjq4XzZizf8SR9ZdQRQ%40mail.gmail.com