Multiple providers, SourceRoot cleared

If you want to use multiple providers, because you may be mirrored/building on multiple systems,

Code like this seems to cause issues
https://github.com/dotnet/sourcelink/blob/main/src/SourceLink.GitHub/build/Microsoft.SourceLink.GitHub.targets#L60-L63

```




```

When I have both GitLab and GitHub providers, I get a warning that no sourcelink information was missing. I think it's because the source root items were cleared and not re-added correctly.