[ci] Update azure pipeline branch variable reference. (sonic-net#612)

Author: liushilongbuaa

azure-pipelines.yml

@@ -46,7 +46,7 @@ variables:
     ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
       value: $(System.PullRequest.TargetBranch)
     ${{ else }}:
-      value: $(Build.SourceBranch)
+      value: $(Build.SourceBranchName)
 
 stages:
 - stage: Build