The author primary signature’s timestamp found a chain building issue: UntrustedRoot: self signed certificate in certificate chain

Update: Check this announcement: https://github.com/NuGet/Announcements/issues/49

At the moment the issue appears to be related to the Debian image.

Switch to an Ubuntu or Alpine based image instead:

FROM mcr.microsoft.com/dotnet/sdk:5.0-focal AS build-env

Follow https://github.com/NuGet/Home/issues/10491 for updates.