Fix the issue for forked branch (Azure#19429)

Co-authored-by: wyunchi-ms <[email protected]>

Author: wyunchi-ms

tools/BuildPackagesTask/Microsoft.Azure.Build.Tasks/FilesChangedTask.cs

@@ -102,8 +102,19 @@ public override bool Execute()
                     //The variable is set in pipeline: "azure-powershell - powershell-core"
                     var client = new GitHubClient(new ProductHeaderValue("Azure"));
                     client.Credentials = new Credentials(Environment.GetEnvironmentVariable("OCTOKITPAT"));
-                    var files = client.PullRequest.Files(RepositoryOwner, RepositoryName, int.Parse(PullRequestNumber))
-                                    .ConfigureAwait(false).GetAwaiter().GetResult();
+                    IReadOnlyList<PullRequestFile> files;
+                    try
+                    {
+                        files = client.PullRequest.Files(RepositoryOwner, RepositoryName, int.Parse(PullRequestNumber))
+                                        .ConfigureAwait(false).GetAwaiter().GetResult();
+                    }
+                    catch (AuthorizationException e)
+                    {
+                        client.Credentials = null;
+                        files = client.PullRequest.Files(RepositoryOwner, RepositoryName, int.Parse(PullRequestNumber))
+                                        .ConfigureAwait(false).GetAwaiter().GetResult();
+                    }
+
                     if (files == null)
                     {
                         return false;