fixes dotnet version

Author: paule96

pipelines/CZ.AspNetCore.EasyAuthAuthentication.yaml

@@ -36,7 +36,9 @@ stages:
             displayName: 'Install dotnet core SDK'
             inputs:
               packageType: 'sdk'
-              useGlobalJson: true
+              # because use global json currently takes always the defined version
+              # and ignores the `rollForward` flag we fall back to this:
+              version: 3.1.x
 
           - task: DotNetCoreCLI@2
             displayName: 'Install GitVersion: $(GitVersion)'