Right now my NuGet restore fails since the project build user doesn\'t have contributor access to the package feed.
/usr/share/dotnet/sdk/3.0.100/NuGe
There was a workaround for this 403 error posted a few hours ago: https://developercommunity.visualstudio.com/content/problem/795493/403-error-during-nuget-restore.html
In short, this seems to affect new projects connecting to a private feed. Here's the suggested work around:
Click "Artifacts" in the project with the failing build
Select the feed you were trying to consume in your build and click the cog in the top right corner
Click "Feed Settings"
Go to the Permissions tab
Click the 3 dots [...] that appeared to the right of the tab
Click "Allow project-scoped builds"
This adds the relevant user permissions that the error the OP posted was complaining about. Hopefully Microsoft will make a proper fix for this soon.
Full credit to Tim Lynch from the developer community page.