automation: use newer Windows base image
It looks like the old base image disappeared. Let's use a newer
image that exists today.
Differential Revision: https://phab.mercurial-scm.org/D6671
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate