Application manifest codebase incorrect when Linux path separator used

### Description

Take these two almost identical commands:
```
dotnet-mage -update -AppManifest Application Files/
dotnet-mage -update -AppManifest Application Files\
```

They both work, but produce different codebase values in the deployment manifest:
```

```

The first one is absolute, breaking the download in every situation except if the deployment manifest is in the root folder of the web site. The second one is relative, functioning correctly. The tool should ideally be smart enough to make the path relative regardless of the path separator used.

### Configuration

.NET 5, Windows, x64

### Regression?

Unknown

### Other information

Here's what appears to be the problematic line of code: https://github.com/dotnet/deployment-tools/blob/309ac6b00368b13ab32456594c655070d2e5bd21/src/clickonce/MageCLI/Mage.cs#L480