.arcconfig

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

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}