[ci] Use official build debian pkg instead and parameterize source br…

…anch (#2079)

#### What I did
1. Use official build debian pkg.
2. parameterize download source branch for future release branch.

azure-pipelines.yml

@@ -29,14 +29,22 @@ stages:
       image: sonicdev-microsoft.azurecr.io:443/sonic-slave-buster:latest
 
     steps:
+    - script: |
+        sourceBranch=$(Build.SourceBranchName)
+        if [[ "$(Build.Reason)" == "PullRequest" ]];then
+          sourceBranch=$(System.PullRequest.TargetBranch)
+        fi
+        echo "Download artifact branch: $sourceBranch"
+        echo "##vso[task.setvariable variable=sourceBranch]$sourceBranch"
+      displayName: "Get correct artifact downloading branch"
     - task: DownloadPipelineArtifact@2
       inputs:
         source: specific
         project: build
-        pipeline: 1
+        pipeline: 142
         artifact: sonic-buildimage.vs
         runVersion: 'latestFromBranch'
-        runBranch: 'refs/heads/master'
+        runBranch: 'refs/heads/$(sourceBranch)'
       displayName: "Download artifacts from latest sonic-buildimage build"
 
     - script: |