The new package output attributes are somewhat experimental, and
provided for compatibility most of all.
We'll see how well this goes before the changes proposed in
https://github.com/NixOS/nix/issues/6507
This ensures just `nix build`-ing the flake doesn't forget to run all
tests. One can still specifiy specific attributes to just build one
thing.
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>