[dotnet] Compute the path to the Xamarin SDK root as a relative path to .NET's root directory. (#19404)

This is useful to compute the path to the Mac's Xamarin SDK from Windows: we
can compute the relative path on Windows, and then just prepend the Mac's path
to .NET.

dotnet/targets/Xamarin.Shared.Sdk.props

@@ -23,6 +23,7 @@
 		<UsingAppleNETSdk>true</UsingAppleNETSdk>
 		<!-- This is the location of the Microsoft.<platform>.Sdk NuGet (/usr/local/share/dotnet/sdk/<version>/Sdks/Microsoft.[iOS/tvOS/watchOS/macOS].Sdk) on the platform the build is running from (Mac or Win) -->
 		<_XamarinSdkRootDirectory>$([MSBuild]::NormalizePath('$(MSBuildThisFileDirectory)', '..'))\</_XamarinSdkRootDirectory>
+		<_XamarinRelativeSdkRootDirectory>$([MSBuild]::EnsureTrailingSlash($([MSBuild]::MakeRelative('$(NetCoreRoot)', '$(_XamarinSdkRootDirectory)'))))</_XamarinRelativeSdkRootDirectory>
 		<!-- This is the location of the Microsoft.<platform>.Sdk NuGet on macOS, this value will be overriden from Windows  -->
 		<_XamarinSdkRootDirectoryOnMac>$(_XamarinSdkRootDirectory)</_XamarinSdkRootDirectoryOnMac>
 		<_XamarinTaskAssembly Condition="'$(_PlatformName)' != 'macOS'">$(_XamarinSdkRootDirectory)\tools\msbuild\iOS\Xamarin.iOS.Tasks.dll</_XamarinTaskAssembly>

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -2041,6 +2041,8 @@
 		<MlaunchPath Condition="'$(MlaunchPath)' == '' And '$(_MlaunchPath)' != ''">$(_MlaunchPath)</MlaunchPath>
 		<MlaunchPath Condition="'$(MlaunchPath)' == ''">$(_XamarinSdkRootDirectory)tools\bin\mlaunch</MlaunchPath>
 		<_MlaunchPath Condition="'$(_MlaunchPath)' == ''">$(MlaunchPath)</_MlaunchPath>
+		<!-- this is the path to mlaunch relative to the root of the .NET installation -->
+		<_RelativeMlaunchPath Condition="'$(_RelativeMlaunchPath)' == ''">$(_XamarinRelativeSdkRootDirectory)tools\bin\mlaunch</_RelativeMlaunchPath>
 	</PropertyGroup>
 
 	<Target Name="ComputeMlaunchInstallArguments" DependsOnTargets="_DetectSdkLocations;_GenerateBundleName;_DetectAppManifest;_ComputeMlaunchInstallArguments" />