[ci] allow download partially succeeded suilds for sonic-swss-pytests artifact (sonic-net#2063)

#### What I did
Currently the latest sonic-swss-pytests has warning in armhf stage. We should allow download artifact from partially succeeded builds.
ref: https://dev.azure.com/mssonic/build/_build?definitionId=15&_a=summary&view=branches

Author: qiluo-msft

.azure-pipelines/test-docker-sonic-vs-template.yml

@@ -41,6 +41,7 @@ jobs:
       artifact: sonic-swss-pytests
       runVersion: 'latestFromBranch'
       runBranch: 'refs/heads/master'
+      allowPartiallySucceededBuilds: true
     displayName: "Download sonic swss pytests"
 
   - checkout: self