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)
This instance will be upgraded to Heptapod 18.5.1 on 2025-11-13 between 17:00 and 18:00 UTC+2
(Don't bother with Windows CI on this, since this script is only manually run)