build(azure): add installation step for .NET 9 SDK

Author: jayharris

src/azure/.pipelines/build.yml

@@ -38,6 +38,11 @@ steps:
   inputs:
     packageType: 'sdk'
     version: '8.x'
+- task: UseDotNet@2
+  displayName: 'Install .NET 9'
+  inputs:
+    packageType: 'sdk'
+    version: '9.x'
 
 - task: DotNetCoreCLI@2
   displayName: 'dotnet restore'