Do not return the tag as branch name for GitLab CI

In case of a tag pipeline, return null as branch name instead of the tag

Author: AlphaYankee

src/GitVersion.BuildAgents/Agents/GitLabCi.cs

@@ -21,7 +21,18 @@ public override string[] GenerateSetParameterMessage(string name, string? value)
         $"GitVersion_{name}={value}"
     ];
 
-    public override string? GetCurrentBranch(bool usingDynamicRepos) => Environment.GetEnvironmentVariable("CI_COMMIT_REF_NAME");
+    public override string? GetCurrentBranch(bool usingDynamicRepos)
+    {
+        // CI_COMMIT_REF_NAME can contain either the branch or the tag
+        // See https://docs.gitlab.com/ee/ci/variables/predefined_variables.html
+
+        // CI_COMMIT_TAG is only available in tag pipelines,
+        // so we can exit if CI_COMMIT_REF_NAME would return the tag
+        if (!string.IsNullOrEmpty(Environment.GetEnvironmentVariable("CI_COMMIT_TAG")))
+            return null;
+
+        return Environment.GetEnvironmentVariable("CI_COMMIT_REF_NAME");
+    }
 
     public override bool PreventFetch() => true;