[Copilot]

The build_mingw job failed during MSYS2 setup when a package mirror returned HTTP 500. Make package installation resilient to transient mirror errors.

• Change: Install packages in a dedicated step and retry up to three times, waiting 10 seconds between attempts. Preserve the existing package list and pacman flags.

for attempt in 1 2 3; do
  if pacman --noconfirm -S --needed --overwrite '*' ...; then
    exit 0
  fi
  # Wait 10 seconds before the next attempt
done