BUILD_SOURCEBRANCH is assumed to be relevant

Author: HHobeck

src/GitVersion.BuildAgents/Agents/AzurePipelines.cs

@@ -17,7 +17,8 @@ public override string[] GenerateSetParameterMessage(string name, string? value)
         $"##vso[task.setvariable variable=GitVersion.{name};isOutput=true]{value}"
     ];
 
-    public override string? GetCurrentBranch(bool usingDynamicRepos) => Environment.GetEnvironmentVariable("BUILD_SOURCEBRANCH");
+    public override string? GetCurrentBranch(bool usingDynamicRepos) => Environment.GetEnvironmentVariable("GIT_BRANCH")
+        ?? Environment.GetEnvironmentVariable("BUILD_SOURCEBRANCH");
 
     public override bool PreventFetch() => true;