packages: 'package-derivation' honors 'system' again.

packages: 'package-derivation' honors 'system' again.

Fixes a regression introduced in
7d873f194ca69d6096d28d7a224ab78e83e34fe1.

Starting from 7d873f194ca69d6096d28d7a224ab78e83e34fe1, running

  guix build -s aarch64-linux sed

on an x86_64-linux machine would return an x86_64-linux machine, whereby
only the top derivation of the graph would be aarch64-linux while all
its dependencies would be x86_64-linux.

* guix/packages.scm (expand-input): Add 'system' parameter and honor it.
(bag->derivation, bag->cross-derivation): Pass SYSTEM to 'expand-input'.
* tests/packages.scm ("package-derivation, different system"): New test.