Merge #20395: ci: Use the previous build worker image in AppVeyor

406097c810 ci: Use the previous build worker image in AppVeyor (Hennadii Stepanov)

Pull request description:

  This is a workaround as the [recent](https://www.appveyor.com/updates/2020/11/14/) Visual Studio 2019 image update breaks our builds.

  This PR is alternative to #20392 due to its build [failure](https://ci.appveyor.com/project/DrahtBot/bitcoin/builds/36314660).

ACKs for top commit:
  MarcoFalke:
    review ACK 406097c810 also seems to pass

Tree-SHA512: f9b722d8e67dd7f0745de6da385064630adb27ecbc0a919be47f62217a2bb7a27a6fa00a7536a24bf17500a77160ca3b92b3c8619047171a6f5198b434015221

.appveyor.yml

@@ -1,6 +1,6 @@
 version: '{branch}.{build}'
 skip_tags: true
-image: Visual Studio 2019
+image: Previous Visual Studio 2019
 configuration: Release
 platform: x64
 clone_depth: 5