Fix syntax to run OneLoc in CI builds (#6858)

Author: zivkan

eng/pipelines/templates/pipeline.yml

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