Add comments about global memoization.

* guix/build-system/gnu.scm (standard-inputs): Add comment about
  misplaced memoization.
* guix/packages.scm (cache): Likewise.

guix/build-system/gnu.scm

@@ -250,6 +250,9 @@ derivations for SYSTEM.  Include propagated inputs in the result."
                            inputs))))
 
 (define standard-inputs
+  ;; FIXME: Memoization should be associated with the open store (as for
+  ;; 'add-text-to-store'), otherwise we get .drv that may not be valid when
+  ;; switching to another store.
   (memoize
    (lambda (system)
      "Return the list of implicit standard inputs used with the GNU Build

guix/packages.scm

@@ -527,6 +527,8 @@ recursively."
 (define (cache package system thunk)
   "Memoize the return values of THUNK as the derivation of PACKAGE on
 SYSTEM."
+  ;; FIXME: This memoization should be associated with the open store, because
+  ;; otherwise it breaks when switching to a different store.
   (let ((vals (call-with-values thunk list)))
     ;; Use `hashq-set!' instead of `hash-set!' because `hash' returns the
     ;; same value for all structs (as of Guile 2.0.6), and because pointer