pin image version in circle CI builds (#6876)

Summary:
somehow the windows-server-2019-vs2019 image changed in a way that made
VS 14 2015 the default. This caused an error when we specify VS 16 2019
as the cmake generator. I could not figure out the right arguments/env
vars to get the latest VS working so pinned the image to the previous
version instead.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6876

Reviewed By: pdillinger

Differential Revision: D21709679

Pulled By: ajkr

fbshipit-source-id: 2d16819ad239b4611fa199547744e1c101dc9da0

.circleci/config.yml

@@ -6,7 +6,7 @@ orbs:
 executors:
   windows-2xlarge:
     machine:
-      image: 'windows-server-2019-vs2019:stable'
+      image: 'windows-server-2019-vs2019:201908-06'
       resource_class: windows.2xlarge
       shell: bash.exe