Build system: need to push symbol packages to NuGet

[Aaronontheweb]

Right now I don't think our current dotnet push command uploads the .snupkg files that contain debugging symbols for Akkatecture. These are needed in order to ensure that SourceLink et al all work properly.

Relevant area of build system:

https://github.com/AfterLutz/Akkatecture/blob/b7c7fd002b2af4002572b029ba58b1356dc7047b/.github/workflows/publish-nuget.yml#L36-L44

[Aaronontheweb]

Might also need to delete this https://github.com/AfterLutz/Akkatecture/blob/b7c7fd002b2af4002572b029ba58b1356dc7047b/Directory.Build.props#L14

I don't think that's necessary if a separate symbol package is being published.