[Bug]: TaskItem normalises path to build platform, not target platform

### Issue Description

I've been working a bit on a bug in the dotnet/sdk#559, and stumbled upon something that _might_ be considered a bug in either Microsoft.Build.Utilities.TaskItem, or Microsoft.Build.Shared.FileUtilities.FixFilePath. One might also argue that it's not a bug, and that it's the usage of it in Microsoft.NET.Build.Containers.Tasks.CreateNewImage that is wrong.

Anyway, the problem is, when publishing dotnet projects to containers, some properties are set on the [CreateNewImage](https://github.com/dotnet/sdk/blob/dd843a86ae8d907fd9c68a42c3dd5c8670d7f74b/src/Containers/Microsoft.NET.Build.Containers/Tasks/CreateNewImage.cs#L14) MSBuild task, that are needed runtime in the built container. So, the paths need to be compatible with the container target platform, not the platform it's built on. The properties in question are (at least) Entrypoint, EntrypointArgs, AppCommand, AppCommandArgs.

However, as the properties are of type `ITaskItem`, implemented by `TaskItem`, the paths are normalised when initialised. And, the normalisation uses the Path.DirectorySeparatorChar of the build platform, not the target.

Would you consider this a bug/shortcoming in the `TaskItem`, or are we/they using it in the wrong way in the MSBuild task?

### Steps to Reproduce

Use a `TaskItem` in an MSBuild task, where you need the directory separator char of a different platform than the one you are building on (e.g. use C:\app\foo.exe when building on *nix)

### Expected Behavior

The item should stay as C:\app\foo.exe

### Actual Behavior

The path is changed to C:/app/foo.exe

### Analysis

https://github.com/dotnet/msbuild/blob/863209471a60e9dfd387ed54d4edb85fd8db57ee/src/Utilities/TaskItem.cs#L80

There is no way to tell either: 1) That you _don't_ want normalisation of directory separator characters, or 2) which style of normalisation you want

### Versions & Configurations

dotnet msbuild --version
17.11.0.25706