automation: make Windows base image name configurable Since automation broke in the middle of the 5.0 release cycle, there's a good chance it will break again in the future. While a robust solution might be to search for all available images and choose the newest one, it does seem useful to be able to explicitly choose the name of the image to find and use so users can opt in to using a different image. This commit implements that functionality. Differential Revision: https://phab.mercurial-scm.org/D6673