Bump windows image version (#220)

Author: jbevain

.yamato/CI.yml

@@ -44,7 +44,7 @@ pack:
   name: Pack
   agent:
     type: Unity::VM
-    image: package-ci/win10:stable
+    image: package-ci/win10:v4
     flavor: b1.large
   commands:
     - npm install upm-ci-utils@latest -g --registry https://artifactory.prd.cds.internal.unity3d.com/artifactory/api/npm/upm-npm