Fix OneLocBuild attempt 2

Author: jeffkl

eng/pipelines/templates/pipeline.yml

@@ -84,7 +84,7 @@ stages:
         LclSource: lclFilesfromPackage
         LclPackageId: 'LCL-JUNO-PROD-NuGetClient'
         MirrorRepo: 'NuGet.Client'
-        MirrorBranch: $[ variables['SourceBranch'] ]
+        MirrorBranch: $(Build.SourceBranch)
         GitHubOrg: 'NuGet'
 
 - stage: StaticSourceAnalysis