HostWithHostFxr is missing NET Framework Reference

**Issue description**
Attempting to build the sample with Visual Studio 2019 at `samples/core/hosting/HostWithHostFxr/` will produce the following error:
`Your project does not reference ".NETFramework,Version=v4.0" framework. Add a reference to ".NETFramework,Version=v4.0" in the "TargetFrameworks" property of your project file and then re-run NuGet restore.`

I'm not quite sure how to resolve this as there does not seem to be such an option under the `NativeHost.vs` project.

**Target framework**

- [x] .NET Core 3.1? (I believe the sample is for 3.1 but I do have 5.0 installed, is there a cause?)
- [ ] .NET Framework
- [ ] .NET Standard

dotnet --info output or About VS info

```console
.NET SDK (reflecting any global.json):
Version: 5.0.200
Commit: 70b3e65d53

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19041
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.200\

Host (useful for support):
Version: 5.0.3
Commit: c636bbdc8a

.NET SDKs installed:
5.0.200 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.25 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.25 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.25 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.12 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
```