Re-enable node reuse for source-build

[dseefeld]

When updating the cli version to an RC version, multiple errors like the following were encountered when building standard:
/src/source-build/packages/microsoft.netcore.compilers/2.8.0-beta2-62719-08/tools/Microsoft.CSharp.Core.targets(52,5): error MSB4062: The "Microsoft.CodeAnalysis.BuildTasks.Csc" task could not be loaded from the assembly /src/source-build/packages/microsoft.netcore.compilers/2.8.0-beta2-62719-08/build/../tools/Microsoft.Build.Tasks.CodeAnalysis.dll. Assembly with same name is already loaded Confirm that the <UsingTask> declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask. [/src/source-build/src/standard/platforms/xamarin.ios/mscorlib.csproj]

To fix the issue, node reuse was turned off in source-build. (see #542) We should try to get all repos to use the same compiler version and then attempt to re-enable note reuse for source-build.