Use header of redirect instead of parsing content (actions#1874)

* Use header of redirect instead of parsing content

* Add exception so we don't hit 404s later

* Fix typo

* Update SelfUpdaterL0.cs

src/Test/L0/Listener/SelfUpdaterL0.cs

@@ -50,9 +50,9 @@ private async Task FetchLatestRunner()
                 var response = await client.SendAsync(new HttpRequestMessage(HttpMethod.Get, "https://github.com/actions/runner/releases/latest"));
                 if (response.StatusCode == System.Net.HttpStatusCode.Redirect)
                 {
-                    var redirect = await response.Content.ReadAsStringAsync();
+                    var redirectUrl = response.Headers.Location.ToString();
                     Regex regex = new Regex(@"/runner/releases/tag/v(?<version>\d+\.\d+\.\d+)");
-                    var match = regex.Match(redirect);
+                    var match = regex.Match(redirectUrl);
                     if (match.Success)
                     {
                         latestVersion = match.Groups["version"].Value;
@@ -63,6 +63,10 @@ private async Task FetchLatestRunner()
                         _packageUrl = $"https://github.com/actions/runner/releases/download/v{latestVersion}/actions-runner-{BuildConstants.RunnerPackage.PackageName}-{latestVersion}.zip";
 #endif
                     }
+                    else
+                    {
+                        throw new Exception("The latest runner version could not be determined so a download URL could not be generated for it. Please check the location header of the redirect response of 'https://github.com/actions/runner/releases/latest'");
+                    }
                 }
             }