For some reason upgrading to 8.0 SDK removes the necessary .deps.json record about Mono.Cecil from illink.deps.json. Workaround which seems to work is to set IncludeProjectsNotInAssetsFileInDepsFile=true in the Mono.Linker and the tasks test projects. But I don't yet understand why.
This is likely related to dotnet/sdk#28963 and other PRs around there.
I'm trying to figure out what's going on. This issue is to track that debt if the workaround gets checked in as part of #3177.