Push nuget nupkg and snupkg together

[mattjohnsonpint]

In #458, I added .snupkg files to the same list of artifacts as .nupkg files. Unfortunately, this didn't work. See getsentry/publish#1857. The issue is that when publishing a .snupkg, nuget will 404 if the corresponding .nupkg has not yet been uploaded.

I though of just sorting the array, but that won't work because we use an array of promises when publishing, which won't guarantee that the first one is complete before the second one starts.

Switching to the original approach I started #458 with: We'll just make sure the .snupkg artifact is downloaded (if there is one), so it will be in the same folder as the .nupkg file. Then dotnet nuget push will automatically see them both when pushing the .nupkg and send the .snupkg only once the .nupkg has successfully been published.

[asottile-sentry]