Fix Mac signing (#18534) (#18539)

* Install .NET SDK for Mac signing

* Install runtime

* Specify 2.1.0

* Install SDK

(cherry picked from commit 23cbb98162)

build/azure-pipelines/darwin/sql-product-build-darwin-signing.yml

@@ -17,6 +17,13 @@ steps:
       mv azuredatastudio-darwin-unsigned.zip azuredatastudio-darwin.zip
     displayName: 'Rename the file'
 
+  - task: UseDotNet@2
+    displayName: 'Install .NET Core sdk for signing'
+    inputs:
+      packageType: sdk
+      version: 2.1.x
+      installationPath: $(Agent.ToolsDirectory)/dotnet
+
   - task: SFP.build-tasks.custom-build-task-1.EsrpCodeSigning@1
     displayName: 'ESRP CodeSigning'
     inputs: