AppVeyor: Add a workaround for build with Visual Studio 2019 on ARM64

Don't use the 2025-11-06 image update (Visual Studio 2019/2022).

Avoid the SDK 10.0.26100.0.

The problem first appeared on 2025-11-07.

The errors were:
C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\um\winnt.h
(6343,27): warning C4013: '_CountOneBits64' undefined; assuming extern
returning int

When it was OK:
cmake version 4.1.0
-- The C compiler identification is MSVC 19.29.30159.0

Current:
cmake version 4.1.0
-- Selecting Windows SDK version 10.0.26100.0 to target Windows 10.0.17763.
-- The C compiler identification is MSVC 19.29.30159.0

[skip ci]

Author: fxlb

.appveyor.yml

@@ -2,6 +2,8 @@ version: '{build}'
 
 clone_depth: 5
 
+image: Previous Visual Studio 2019
+
 branches:
   except:
     - coverity_scan