Fixes <https://bugs.gnu.org/42327>.
Reported by Jan Nieuwenhuizen <janneke@gnu.org>.
This is a followup to f52fbf7094.
* guix/packages.scm (bag-transitive-inputs, bag-transitive-build-inputs)
(bag-transitive-host-inputs, bag-transitive-target-inputs): Parameterize
%CURRENT-SYSTEM in addition to %CURRENT-TARGET-SYSTEM.
* tests/packages.scm ("package->bag, sensitivity to %current-system"):
New test.