ci: windows: Use the same image for VS and MinGW tasks

ci: windows: Use the same image for VS and MinGW tasks

The VS and MinGW Windows images have been merged, to reduce the space needed
for images. Before 98811323c8e the split helped boot performance, but now that
we are using VMs that doesn't appear to be the case anymore.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: http://postgr.es/m/CAN55FZ2kWYjPd7uUC5QswrB3tfVJDiURqC%2BMGM6a3oeev%3DVgOA%40mail.gmail.com
Backpatch: 15-, where CI was added