[xharness] Get new mlaunch for beta 4.

tests/xharness/Harness.cs

@@ -89,7 +89,8 @@ public string XcodeRoot {
 		string DownloadMlaunch ()
 		{
 			// Just hardcode this for now. We should be able to switch to a shipped version of XS soon.
-			var mlaunch_url = "https://bosstoragemirror.blob.core.windows.net/public-builder/mlaunch";
+			// NOTE: the filename part in the url must be unique so that the caching logic works properly.
+			var mlaunch_url = "http://bosstoragemirror.blob.core.windows.net/public-builder/mlaunch-ade5bd316053a4f747df13fd1e0386544ad223c5";
 			var mlaunch_path = Path.Combine (Path.GetTempPath (), Path.GetFileName (mlaunch_url), "mlaunch");
 			if (File.Exists (mlaunch_path))
 				return mlaunch_path;