Adding nuget installer.

Author: mijpeterson

.github/workflows/pr-azure-pipelines.yml

@@ -66,14 +66,17 @@ extends:
             displayName: Use .NET Core sdk 8.x
             inputs:
               version: 8.x
+          - task: NuGetToolInstaller@0
+            displayName: Use NuGet 6.x
+            inputs:
+              versionSpec: 6.x
           - task: DotNetCoreCLI@2
             displayName: Install dependencies
             inputs:
               command: restore
               feedsToUse: select
               vstsFeed: $(VSTS_FEED_ID)
               includeNuGetOrg: false
-              nugetConfigPath: 'nuget.config'
           # 1ES PT requires explicit build task for Roslyn analysis. Auto-injected Roslyn task will use build logs from this build.
           - task: DotNetCoreCLI@2
             displayName: Build projects