grafts: Preserve the cache across recursive calls.

Before this commit, we'd lose the cache across recursive calls to
'cumulative-grafts', which isn't great performance-wise.

This bug was already present before
d38bc9a9f6.

* guix/grafts.scm (with-cache): In the miss case, call 'current-state'
after EXP has been evaluated.

guix/grafts.scm

@@ -221,7 +221,8 @@ available."
       ((_ . result)                               ;cache hit
        (return result))
       (#f                                         ;cache miss
-       (mlet %state-monad ((result (begin exp ...)))
+       (mlet %state-monad ((result (begin exp ...))
+                           (cache  (current-state)))
          (mbegin %state-monad
            (set-current-state (vhash-consq key result cache))
            (return result)))))))