Add service connection for NugetAuthenticate (#778)

Add service connection conditionally
Update "Install .NET SDK" action to use the contents of global.json to determine the version of the SDK to install

Author: Phil Allen

azure-pipelines/build.yml

@@ -4,6 +4,9 @@ steps:
 
 - task: UseDotNet@2
   displayName: Install .NET Core SDK
+  inputs:
+    packageType: sdk
+    useGlobalJson: true
 
 - ${{ if eq(variables['system.collectionId'], '011b8bdf-6d56-4f87-be0d-0092136884d9') }}:
   - template: /azure-pipelines/azure-pipeline.microbuild.before.yml@self
@@ -13,6 +16,9 @@ steps:
 
 - task: NuGetAuthenticate@1
   displayName: 'Authenticate with NuGet'
+  inputs:
+    ${{ if eq(variables['system.collectionId'], '011b8bdf-6d56-4f87-be0d-0092136884d9') }}:
+      nuGetServiceConnections: azure-public/readonly
 
 - task: PowerShell@2
   displayName: 'dotnet restore'