[xaprepare] use $(XAPackagesDir) for NuGet folder

[jonathanpeppers]

After returning from the holidays, xamarin-android failed to build (-t:Prepare) with:

Step Xamarin.Android.Prepare.Step_InstallDotNetPreview failed: 7zip executable 'D:\7-zip.commandline\18.1.0\tools\x64\7za.exe' not found
System.InvalidOperationException: Step Xamarin.Android.Prepare.Step_InstallDotNetPreview failed: 7zip executable 'D:\7-zip.commandline\18.1.0\tools\x64\7za.exe' not found
---> System.InvalidOperationException: 7zip executable 'D:\7-zip.commandline\18.1.0\tools\x64\7za.exe' not found
    at Xamarin.Android.Prepare.ToolRunner..ctor(Context context, Log log, String toolPath) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\ToolRunners\ToolRunner.cs:line 57
    at Xamarin.Android.Prepare.SevenZipRunner..ctor(Context context, Log log, String toolPath) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\ToolRunners\SevenZipRunner.cs:line 19
    at Xamarin.Android.Prepare.Utilities.Unpack(String fullArchivePath, String destinationDirectory, Boolean cleanDestinatioBeforeUnpacking) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\Application\Utilities.cs:line 142
    at Xamarin.Android.Prepare.Step_InstallDotNetPreview.InstallDotNetAsync(Context context, String dotnetPath, String version, Boolean runtimeOnly) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\Steps\Step_InstallDotNetPreview.cs:line 250
    at Xamarin.Android.Prepare.Step_InstallDotNetPreview.Execute(Context context) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\Steps\Step_InstallDotNetPreview.cs:line 24
    at Xamarin.Android.Prepare.Step.Run(Context context) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\Application\Step.cs:line 42
    at Xamarin.Android.Prepare.Scenario.Run(Context context, Log log) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\Application\Scenario.cs:line 37
    --- End of inner exception stack trace ---
    at Xamarin.Android.Prepare.Scenario.Run(Context context, Log log) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\Application\Scenario.cs:line 48
    at Xamarin.Android.Prepare.Context.Execute() in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\Application\Context.cs:line 511
    at Xamarin.Android.Prepare.App.Run(String[] args) in D:\src\xamarin-android\build-tools\xaprepare\xaprepare\Main.cs:line 162

I am using %NUGET_PACKAGES% on my local machine to point to a DevDrive at D:\.nuget\packages\, for some reason this is breaking the build?

Previously we were using logic such as:

return Path.GetFullPath (
    Path.Combine (
        ctx.Properties.GetRequiredValue (KnownProperties.PkgXamarin_LibZipSharp),
        "..",
        ".."
    )
);

Instead of using $(PkgXamarin_LibZipSharp), let's use $(XAPackagesDir) as logic in Configuration.props was added in 66a0389 to consider %NUGET_PACKAGES%.