Update nightly pipeline with missing sdks (#1512)

Author: daveMueller

eng/azure-pipelines-nightly.yml

@@ -12,6 +12,16 @@ steps:
     version: 5.0.401
   displayName: Install .NET Core SDK 5.0.401
 
+- task: UseDotNet@2
+  inputs:
+    version: 6.0.408
+  displayName: Install .NET Core SDK 6.0.408
+
+- task: UseDotNet@2
+  inputs:
+    version: 7.0.203
+  displayName: Install .NET Core SDK 7.0.203
+
 - task: NuGetAuthenticate@0
   displayName: Authenticate with NuGet feeds