ci: Use windows VMs instead of windows containers

ci: Use windows VMs instead of windows containers

So far we have used containers for testing windows on cirrus-ci. Unfortunately
they come with substantial overhead: First, the container images are pulled
onto the host on-demand. Due to the large size of windows containers, that
ends up taking nearly 4 minutes. Secondly, IO is slow, leading to CI runs
taking long.

Thus switch to windows VMs, improving windows CI times by well over 2x.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/211afb88-6df6-b74d-f1b7-84b5f21ad875@gmail.com
Backpatch: 15-, where CI was added