AppVeyor: Use 'zopfli' to generate '.gz' source tarballs
AppVeyor: Use 'zopfli' to generate '.gz' source tarballs

Previously, 'gzip' ended up getting used, as MSYS2 doesn't have 'pigz',
and 'zopfli' wasn't installed.

Also, make sure 'zopfli' is on the path, when needed.

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