Update build pipeline to use automatic pool provider (#297)

Author: twsouthwick

azure-pipelines.yml

@@ -10,6 +10,9 @@ variables:
   - name: _DotNetArtifactsCategory
     value: .NETCore
 
+  # Variables to automatically handle build pools
+  - template: /eng/common/templates/variables/pool-providers.yml
+
   # Variables for public PR builds
   - ${{ if or(eq(variables['System.TeamProject'], 'public'), in(variables['Build.Reason'], 'PullRequest')) }}:
     - name: _HelixSource
@@ -54,10 +57,10 @@ stages:
       - job: Windows
         pool:
           ${{ if eq(variables['System.TeamProject'], 'public') }}:
-            name: NetCore-Public
+            name: $(DncEngPublicBuildPool)
             demands: ImageOverride -equals 1es-windows-2019-open
           ${{ if ne(variables['System.TeamProject'], 'public') }}:
-            name: NetCore1ESPool-Internal
+            name: $(DncEngInternalBuildPool)
             demands: ImageOverride -equals 1es-windows-2019
         variables:
         - name: _HelixBuildConfig