store: 'mapm/accumulate-builds' preserves '%current-target-system'.

Fixes <https://bugs.gnu.org/41182>.

* guix/store.scm (mapm/accumulate-builds): Pass #:system and #:target to
'run-with-store'.
* tests/store.scm ("mapm/accumulate-builds, %current-target-system"):
New test.
* tests/guix-pack.sh: Add 'guix pack -d --target' test.