Merge #19839: Set appveyor VM version to previous Visual Studio 2019 release

3ec8f4c Set appveyor vm version to previous Visual Studio 2019 release. (Aaron Clauson)

Pull request description:

  The Visual Studio 2019 Appveyor [update](https://www.appveyor.com/updates/2020/08/29/) includes a bump of `cmake` to `3.18.2`.

  The `berkeleydb` dependency from the pegged `vcpkg` commit now fails to build with the `cmake` update.

  Setting the Appveyor VM back to the previous version should fix the immediate issue while a solution is identified for updating the `berkeleydb` build configuration.

ACKs for top commit:
  hebasto:
    ACK 3ec8f4c, AppVeyor build is ok now.

Tree-SHA512: 61e10d0260c1c51dc6c552c857b3013f55ee324e26b1229bf7ef6f14c3e296762148db66a994ecc33fdc43e81592a7f8442d17b407964edb774b482d84735757

Author: MarcoFalke

.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