Use TargetBranch as a fallback

Author: cccs-rs

pipelines/azure-test.yaml

@@ -8,7 +8,7 @@ pool:
 
 variables:
   # Try to checkout the matching branch, if the command fails, don't care.
-  BRANCH_NAME: $[coalesce(variables['System.PullRequest.SourceBranch'], replace(variables['Build.SourceBranch'], 'refs/heads/', ''))]
+  BRANCH_NAME: $[coalesce(variables['System.PullRequest.SourceBranch'], variables['System.PullRequest.TargetBranch'], replace(variables['Build.SourceBranch'], 'refs/heads/', ''))]
 
 resources:
   containers: