contrib: move the MinGW download location to mercurial-scm.org
(Don't bother with Windows CI on this, since this script is only manually run)
Meet users and developers of Mercurial at the London Sprint, May 27-29th https://mercurial-scm.org/news/2026/0001-london-sprint
(Don't bother with Windows CI on this, since this script is only manually run)