Static file "static" configuration

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the bug

I expected to be able to configure static file wwwroot directory (_site in my case) by setting Environment.ContentRootPath/WebRootPath but I was only able to do it with a file provider (physical).

### Expected Behavior

```
using var app = WebApplication.Create();
app.Urls.Add($"http://localhost:4200"); // todo: config

// works but not desired:
// app.Environment.WebRootFileProvider = new PhysicalFileProvider(configuration.Output.Location);

// desired but does not work
app.Environment.ContentRootPath = "/opt/foo/test";
app.Environment.WebRootPath = "_site"; // also tested an absolute path

app.UseDefaultFiles();
app.UseStaticFiles();

logger.LogInformation("Started server on {Urls}", app.Urls);
app.Run();
```

What's weird is that it logs the right folder but that it does not exist (whereas I can always "ls" it)

### Steps To Reproduce

Just create a minimal aspnet.core project with ^^ code

### Exceptions (if any)

-

### .NET Version

8.0.107

### Anything else?

```
SDK .NET :
Version: 8.0.107
Commit: 1bdaef7265
Workload version: 8.0.100-manifests.43c23f91

Environnement d'exécution :
OS Name: zorin
OS Version: 17
OS Platform: Linux
RID: ubuntu.22.04-x64
Base Path: /usr/lib/dotnet/sdk/8.0.107/

Charges de travail .NET installées :
Workload version: 8.0.100-manifests.43c23f91
Il n'y a aucune charge de travail installée à afficher.

Host:
Version: 8.0.7
Architecture: x64
Commit: 2aade6beb0

.NET SDKs installed:
8.0.107 [/usr/lib/dotnet/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.7 [/usr/lib/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 8.0.7 [/usr/lib/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
None

Environment variables:
Not set

global.json file:
/opt/rmannibucau/dev/SBSharp/global.json

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download
```