appveyor: switch to Previous Visual Studio 2019 (VS issues after update)

appveyor: switch to Previous Visual Studio 2019 (VS issues after update)

After update [1] of the Visual Studio 2019 image cmake unable to find
path to the c compiler [2]:

    cmake : CMake Error at CMakeLists.txt:47 (project):
    At line:1 char:1
    + cmake -G 'Visual Studio 16 2019' -A x64 ..
    + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        + CategoryInfo          : NotSpecified: (CMake Error at ...t:47 (project)::String) [], RemoteException
        + FullyQualifiedErrorId : NativeCommandError

      The CMAKE_C_COMPILER:
        C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.23.28105/bin/Hostx64/x64/cl.exe
      is not a full path to an existing compiler tool.

  [1]: https://github.com/appveyor/ci/issues/3231
       https://www.appveyor.com/updates/2019/12/09/
  [2]: https://ci.appveyor.com/project/libevent/libevent/builds/29431286/job/a68h7dn9rcride9g