trying to downgrad the .NET sdk

.az-pipelines/windows-build-sign.yml

@@ -21,6 +21,12 @@ steps:
   #    set DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
   #    dotnet tool install --global AzureSignTool --version 2.0.17
   #  displayName: 'install AzureSignTool'
+- task: UseDotNet@2
+  displayName: 'Use .NET Core SDK 3.x'
+  inputs:
+    packageType: sdk
+    version: 3.x
+    installationPath: $(Agent.ToolsDirectory)/dotnet
 
 - script: |
     git clone https://github.com/vcsjones/AzureSignTool.git