Upgrade AppVeyor image

This fixes the MSYS2 signing keys and also upgrades pacman to
support their .zst packages.

See https://www.msys2.org/news/#2020-06-29-new-packagers

.appveyor.yml

@@ -22,6 +22,9 @@ skip_tags: true
 #    - docs/*
 #    - '**/*.html'
 
+# Appveyor Windows images are based on Visual studio version
+image: Visual Studio 2019
+
 # We use Mingw/Msys, so use pacman for installs
 install:
   - set HOME=.