Fixes <https://bugs.gnu.org/44760>.

Previously, the 'register-path' call would re-traverse ITEM to compute
its nar hash, even though that hash is already known in the initial
store.  This patch also avoids repeated opening/closing of the
database.

* guix/store/database.scm (call-with-database): Export.
* guix/scripts/system.scm (copy-item): Add 'db' parameter.  Call
'sqlite-register' instead of 'register-path'.
(copy-closure): Remove redundant call to 'references*'.  Call
'call-with-database' and pass the database to 'copy-item'.