Change the `nix-build` logic for linking/printing the output paths to allow for
some outputs to be missing. This might happen when the toplevel
derivation didn't have to be built, either because all the required
outputs were already there, or because they have all been substituted.

Simple test to ensure that `nix-build && nix-collect-garbage &&
nix-build -j0` works as it should

Once a build is done, get back to the original derivation, and register
all the newly built outputs for this derivation.

This allows Nix to work properly with derivations that don't have all
their build inputs available − thus allowing garbage collection and
(once it's implemented) binary substitution