NuGet.CommandLine: Platform check on PushCommand_PushToFileSystemSourceUncStyle is incorrect

NuGet.CommandLine: Platform check on PushCommand_PushToFileSystemSourceUncStyle is incorrect. Fix this

```
        // UNC only works in Windows. So skip this test if we're running on Unix,
        if (Path.DirectorySeparatorChar == '/')
        {
            return;
        }
```
