gexp: Fix expansion for (file-append (local-file ...) ...).

Fixes <https://bugs.gnu.org/41527>.
Regression introduced in d03001a3.

* guix/gexp.scm (lower+expand-object): When LOWERED is not a struct and
EXPAND is true, call EXPAND.
* tests/gexp.scm ("file-append, raw store item"): New test.