Symbol publishing is failing from CI

[nblumhardt]

Published CI builds are failing with:

Deploying using NuGet provider
Publishing Serilog.AspNetCore.3.4.0-dev-00176.nupkg to https://www.nuget.org/api/v2/package...OK
Publishing Serilog.AspNetCore.3.4.0-dev-00176.symbols.nupkg to https://nuget.smbsrc.net/api/v2/package...
Error publishing package. NuGet server returned 500: Internal Server Error

This issue used to plague us some time back. I've disabled symbol publishing for now with skip_symbols in appveyor.yml, but it's likely we need to switch to the new snupkg format.