Feature request: trix build options to control number of used cores

[sergv]

The original nix build has --cores and --jobs to limit how much building happens at any one time.

The underlying nix-build also has the same options. It seems like a good and not too costly idea to have trix accept --cores and --jobs and pass them to the underlying nix-build invocation.

Also having --keep-going in trix build would be good for long-running builds like the ones that occur when NixOS version updates.