Fix setting User-Agent for dotnet scenarios (#620)

Author: dfederm

src/UniversalPackages/DownloadUniversalPackages.cs

@@ -629,7 +629,7 @@ private string GetArtifactToolReleaseInfoUrl(string osName, string arch)
         string json = webClient.DownloadString(ReleaseInfoUrl);
 #else
         using HttpClient httpClient = new HttpClient();
-        httpClient.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("Microsoft.Build.UniversalPackages"));
+        httpClient.DefaultRequestHeaders.TryAddWithoutValidation("User-Agent", "Microsoft.Build.UniversalPackages");
         string json = httpClient.GetStringAsync(ReleaseInfoUrl).GetAwaiter().GetResult();
 #endif