appveyor: Re-install MinGW from OSDN.net
appveyor: Re-install MinGW from OSDN.net

Directly upgrading from the existing [sourceforge] installation to the
new [OSDN] one fails. To work around that, the existing installation is
now deleted, and the new one is installed in its place by unpacking a
new version of 'mingw-get', and installing the needed packages through
it.

TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf