[xaprepare] skip download progress for dotnet-install script (#9053)

We are getting the error pretty early on during the build:

    Downloading dotnet-install script...
    Warning: Failed to obtain dotnet-install script size from URL 'https://dot.net/v1/dotnet-install.sh'. HTTP status code: InternalServerError (500)a
      -> https://dot.net/v1/dotnet-install.sh
        downloading dotnet-install.sh

It seems the code is timing out on the call:

    await httpClient.GetAsync (url, HttpCompletionOption.ResponseHeadersRead);

And then the code has some retry logic that eventually returns:

    return (false, 0, HttpStatusCode.InternalServerError);

I am unable to reproduce this locally on either Mac or Windows.

Reworking this code to just download the file with no progress appears
to solve the problem on Azure DevOps build agents.

build-tools/xaprepare/xaprepare/Application/DownloadStatus.cs

@@ -10,6 +10,8 @@ class DownloadStatus
 		const uint DefaultUpdateInterval = 1000;
 		readonly object updateLock = new object ();
 
+		public static readonly DownloadStatus Empty = new DownloadStatus (0, _ => { });
+
 		ConcurrentQueue<ulong> byteSnapshots;
 		Stopwatch watch;
 		Action<DownloadStatus> updater;

build-tools/xaprepare/xaprepare/Steps/Step_InstallDotNetPreview.cs

@@ -78,23 +78,13 @@ async Task<bool> DownloadDotNetInstallScript (Context context, string dotnetScri
 
 			Log.StatusLine ("Downloading dotnet-install script...");
 
-			(bool success, ulong size, HttpStatusCode status) = await Utilities.GetDownloadSizeWithStatus (dotnetScriptUrl);
-			if (!success) {
-				if (status == HttpStatusCode.NotFound) {
-					Log.WarningLine ($"dotnet-install URL '{dotnetScriptUrl}' not found.");
-				} else {
-					Log.WarningLine ($"Failed to obtain dotnet-install script size from URL '{dotnetScriptUrl}'. HTTP status code: {status} ({(int) status})");
-				}
-
-				if (File.Exists (dotnetScriptPath)) {
-					Log.WarningLine ($"Using cached installation script found in '{dotnetScriptPath}'");
-					return true;
-				}
+			if (File.Exists (dotnetScriptPath)) {
+				Log.WarningLine ($"Using cached installation script found in '{dotnetScriptPath}'");
+				return true;
 			}
 
-			DownloadStatus downloadStatus = Utilities.SetupDownloadStatus (context, size, context.InteractiveSession);
 			Log.StatusLine ($"  {context.Characters.Link} {dotnetScriptUrl}", ConsoleColor.White);
-			await Download (context, dotnetScriptUrl, tempDotnetScriptPath, "dotnet-install", Path.GetFileName (dotnetScriptUrl.LocalPath), downloadStatus);
+			await Utilities.Download (dotnetScriptUrl, tempDotnetScriptPath, DownloadStatus.Empty);
 
 			if (File.Exists (tempDotnetScriptPath)) {
 				Utilities.CopyFile (tempDotnetScriptPath, dotnetScriptPath);