[Linux] Properly parse single-digit Ubuntu version (#5463)

Fixes: #4929

Some releases of Ubuntu use only a single digit for their version:

	   OS type: Linux
	   OS name: Linux Mint
	OS release: 20
	   OS bits: x86_64
	 CPU count: 2

This would cause `xaprepare` to error out on these platforms:

	$ make prepare
	…
	  Error: Unable to parse string '20' as a valid Ubuntu release version

	Failed to initialize operating system support
	System.InvalidOperationException: Failed to initialize operating system support
	  at Xamarin.Android.Prepare.OS.Init () [0x00024] in /home/zola/Desktop/xamarin-android/build-tools/xaprepare/xaprepare/OperatingSystems/OS.cs:243 
	  at Xamarin.Android.Prepare.Context.Init (System.String scenarioName) [0x0022d] in /home/zola/Desktop/xamarin-android/build-tools/xaprepare/xaprepare/Application/Context.cs:766 
	  at Xamarin.Android.Prepare.App.Run (System.String[] args) [0x00793] in /home/zola/Desktop/xamarin-android/build-tools/xaprepare/xaprepare/Main.cs:153

	make: *** [Makefile:204: prepare] Error 1

If `Version.TryParse()` fails to parse `OS release`, add a fallback
which uses `Int32.TryParse()`.  This should allow `make prepare` to
run on these Linux platforms.

Additionally, don't hardcode `Ubuntu` into the error message, use the
actual (derived) distro name instead.

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

@@ -749,6 +749,7 @@ public async Task<bool> Init (string? scenarioName = null)
 
 			Log.StatusLine ();
 			Log.StatusLine ("   OS type: ", OS.Type, tailColor: Log.InfoColor);
+			Log.StatusLine (" OS flavor: ", OS.Flavor, tailColor: Log.InfoColor);
 			Log.StatusLine ("   OS name: ", OS.Name, tailColor: Log.InfoColor);
 			Log.StatusLine ("OS release: ", OS.Release, tailColor: Log.InfoColor);
 			Log.StatusLine ("   OS bits: ", OS.Architecture, tailColor: Log.InfoColor);

build-tools/xaprepare/xaprepare/ConfigAndData/Dependencies/Linux.Ubuntu.cs

@@ -42,8 +42,12 @@ protected override bool InitOS ()
 		{
 			Version ubuntuRelease;
 			if (!Version.TryParse (Release, out ubuntuRelease)) {
-				Log.ErrorLine ($"Unable to parse string '{Release}' as a valid Ubuntu release version");
-				return false;
+				if (Int32.TryParse (Release, out int singleNumberVersion)) {
+					ubuntuRelease = new Version (singleNumberVersion, 0);
+				} else {
+					Log.ErrorLine ($"Unable to parse string '{Release}' as a valid {Name} release version");
+					return false;
+				}
 			}
 			UbuntuRelease = ubuntuRelease;