[msbuild][mac] Fix references netstandard libs from XM/Full (#2656)

This is to handle the case of XM/Full project referencing a netstandard project.

For XM/Full, $(TargetFrameworkDirectory) is `lib/mono/4.5` which has
`netstandard.dll`.  This causes ImplicitlyExpandNETStandardFacades to
skip expanding assuming that when ImplicitlyExpandDesignTimeFacades
expands facades, `netstandard.dll` would also get referenced.

But if the XM project does NOT have any System.Runtime facades, then
ImplicitlyExpandDesignTimeFacades will not expand the facades and so we
end up with no `netstandard.dll` reference!

With `$(NETStandardInbox) == false`,
`ImplicitlyExpandNETStandardFacades` behaves as if `netstandard.dll` was
not available in the framework directories and will expand the facades
if required.

Partially fixes bxc #58504 .

msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.Common.targets

@@ -87,6 +87,21 @@ Copyright (C) 2014 Xamarin. All rights reserved.
 
 	<PropertyGroup Condition="'$(TargetFrameworkName)' == 'Full'">
 		<AssemblySearchPaths>$(XamarinMacFrameworkRoot)/lib/reference/full;$(XamarinMacFrameworkRoot)/lib/mono;$(AssemblySearchPaths)</AssemblySearchPaths>
+
+		<!--
+		     This is to handle the case of XM/Full project referencing a netstandard project.
+
+		     For XM/Full, $(TargetFrameworkDirectory) is `lib/mono/4.5` which has `netstandard.dll`.
+		     This causes ImplicitlyExpandNETStandardFacades to skip expanding assuming that when ImplicitlyExpandDesignTimeFacades
+		     expands the facades, `netstandard.dll` would get referenced too.
+
+		     But if the XM project does NOT have any System.Runtime facades, then ImplicitlyExpandDesignTimeFacades will not expand
+		     the facades and so we end up with no `netstandard.dll` reference!
+
+		     With `$(NETStandardInbox) == false`, `ImplicitlyExpandNETStandardFacades` behaves as if `netstandard.dll` was not
+		     available in the framework directories and will expand the facades if required.
+		-->
+		<NETStandardInbox Condition="'$(NETStandardInbox)' == ''">false</NETStandardInbox>
 	</PropertyGroup>
 
 	<!-- Do not resolve from the GAC in Modern or Full unless allow-unsafe-gac-resolution is passed in -->