Improve UX around evaluating all of nixpkgs

[getchoo]

Nixpkgs is very large. We should be restricting evaluation of installables to those of the current system (or by a --system flag, see guix weather) to lighten the load of running this command, give the user a better idea of what's cached for their system instead of all systems, and avoid evaluation errors with packages on more exotic systems

And speaking of evaluation errors: like guix weather, currently we just hard fail at evaluation failure. This isn't great behavior and something we should improve on (especially as evaluation failures are seemingly more common in nixpkgs than guix), but I'm not sure how to go about it yet. My current idea is to filter out failures and report a count of how many installables are not considered in the final result, but that could also be sort of weird behavior :/