AppVeyor: Use MinGW-builds instead of MSYS2 MinGW

AppVeyor: Use MinGW-builds instead of MSYS2 MinGW

... to avoid backward compatibility issues with GCC 4-6 MinGW
toolchains.  Apparently GCC 7+ MinGW toolchains introduce a link-time
dependency with internal MinGW CRT functions that are meant to provide
compatibility with Microsoft's Universal CRT (ucrt) library, but those
internal functions are not available in GCC 4-6 MinGW toolchains.  This
made it impossible to use the official builds of libjpeg.a and
libturbojpeg.a with GCC 4-6 MinGW toolchains (a fatal link error--
"undefined reference to '__imp___acrt_iob_func'"-- occurred.)

This problem was not immediately apparent after switching to the MSYS2
implementation of MinGW (d6d7b539682abb50b04207abd21726573c0fb845)
because, for a while, MSYS2 was still using GCC 5 and 6.

Refer to libjpeg-turbo/libjpeg-turbo#382