MSBuild::NormalizePath does not correctly canocalize paths containing back-slashes on xplat

[pranavkm]

Steps to reproduce

Project file

<Project>
    <Target Name="TestNormalize">
        <PropertyGroup>
            <Dir1>$([MSBuild]::NormalizePath('$(MSBuildThisFileDirectory)..\test'))</Dir1>
            <Dir2>$([MSBuild]::NormalizePath('$(MSBuildThisFileDirectory)../test'))</Dir2>
        </PropertyGroup>
        
        <Message Text="$(Dir1) $(Dir2)" Importance="High" />
    </Target>
</Project>

On Windows, the result is:

D:\test D:\test

On Ubuntu, this is what I get:

/mnt/d/utemp/../test /mnt/d/test

This is not a problem since the File I/O APIs would resolve either paths. However, in this particular I was trying to remove an item from an ItemGroup using it's canonicalized path.