provisionary fix CI build

Author: maurei

Build.ps1

@@ -93,6 +93,14 @@ function CreateNuGetPackage {
     CheckLastExitCode
 }
 
+function LoadOpenApiBranchIfRequired {
+    if ($env:APPVEYOR_REPO_BRANCH -eq "openapi") {
+        git fetch origin openapi:openapi
+    }
+}
+
+LoadOpenApiBranchIfRequired
+
 dotnet tool restore
 CheckLastExitCode