Nix - Show cached status or similar when dependencies are already satisfied

[rdnt]

bob currently prints 2 lines relevant to building nix dependencies, even if the deps are already satisfied. It should probably indicate that the build was cached, or even include the message in the build task output and show it as a virtual build task.

Alternatively, the 2 lines can be simplified into one, e.g. Building Nix dependencies... done (no newline).