appveyor: Use VS2019 build worker image for MinGW
appveyor: Use VS2019 build worker image for MinGW

The Visual Studio 2019 build worker image has MinGW installed with the
newer mingw-get from OSDN.net. This means the MinGW toolchain can be
upgraded, without having to install the whole thing from scratch.

(This basically reverts changes made in commits
ac9a38d0a13f56a00c2b3fa3f39aa2bebe1361e8 and
30b3181fc7373247246318033aab54335c027da2)

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