grafts: Actually cache grafts during the derivation DAG traversal.

This fixes a regression introduced in
d38bc9a9f6 whereby the cache was
effectively disabled.

Reported by Thomas Danckaert <thomas.danckaert@gmail.com>.

* guix/grafts.scm (with-cache): In the cache miss case, wrap body in
'mbegin'.

guix/grafts.scm

@@ -222,8 +222,9 @@ available."
        (return result))
       (#f                                         ;cache miss
        (mlet %state-monad ((result (begin exp ...)))
+         (mbegin %state-monad
            (set-current-state (vhash-consq key result cache))
-         (return result))))))
+           (return result)))))))
 
 (define* (cumulative-grafts store drv grafts
                             references