mdbx-ci: allow builds using MSVC-2019 to fail due to internal compiler errors.

At least the MSVC 19.27.29111.0 hang up during code generation/optimization due to its own internal errors.

Related to https://github.com/erthink/libmdbx/issues/116

Change-Id: I016769b38dea800da5450bd33fb241f145ff24da

appveyor.yml

@@ -41,6 +41,16 @@ platform:
 - Win32
 - x64
 
+# MSVC-2019 may hang up during code generation/optimization due to its own internal errors.
+matrix:
+  allow_failures:
+    - image: Visual Studio 2019
+      configuration: Release
+
+# Enable RDP for troubleshooting
+init:
+  - ps: iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
+
 before_build:
 - git clean -x -f -d
 - git submodule sync