Cake Issue when addin uses nullable reference types

Repro for cake-build/cake#4197

This repository reproduces an issue in Cake that ocurrs, when an addin enables nullable referency types.

If a nullable annotation is used in an Cake alias, that is imported into a Cake script, running the script fails with an error like

(3423,57): error CS0234: The type or namespace name 'NullableAttribute' does not exist in the namespace 'System.Runtime.CompilerServices' (are you missing an assembly reference?)
(3423,57): error CS0234: The type or namespace name 'Nullable' does not exist in the namespace 'System.Runtime.CompilerServices' (are you missing an assembly reference?)

Run the repro

Build the sample addin by running

dotnet build --configuration Debug "ExampleAddin/ExampleAddin.csproj"

Run the Cake script that uses the alias

cd "CakeScript"
dotnet tool restore 
dotnet cake