[ci]: Fix the wrong branch in build step "Download sonic-buildimage.vs artifact" (#12272)

Author: qiluo-msft

azure-pipelines.yml

@@ -101,7 +101,7 @@ stages:
         pipeline: 9
         artifact: sonic-swss-common.amd64.ubuntu20_04
         runVersion: 'latestFromBranch'
-        runBranch: 'refs/heads/master'
+        runBranch: 'refs/heads/202012'
       displayName: "Download sonic swss common deb packages"
 
     - task: DownloadPipelineArtifact@2