packages: Ensure bags are insensitive to '%current-target-system'.

Fixes a bug whereby a bag's transitive dependencies would depend on the
global '%current-target-system' value.

Partly fixes <https://issues.guix.gnu.org/41182>.

* guix/packages.scm (bag-transitive-inputs)
(bag-transitive-build-inputs, bag-transitive-target-inputs):
Parameterize '%current-target-system'.
* tests/packages.scm ("package->bag, sensitivity to %current-target-system"):
New test.