sdk: Build fails on missing generated files

Hi,

I started to see this in one of my Azure Pipeline builds for the specific .AssemblyAttributes file but random projects:

CSC : error CS2001: Source file 'D:\a\1\s\src\..\bld\obj\AnyCPU_Release\WorkItems\.AssemblyAttributes' could not be found. [D:\a\1\s\src\WorkItems\WorkItems.csproj]

I observed that this started to happen after net5.0 got installed by default in windows-latest machines.

environment: windows-latest azure pipeline dotnet versions: 2.1.x, 3.1.x, 5.0.x (versions pre-installed).

cc: @jasonmalinowski