guix: store: Add ensure-path.

* guix/store.scm (ensure-path): New procedure.

guix/store.scm

@@ -113,6 +113,7 @@
             build
             query-failed-paths
             clear-failed-paths
+            ensure-path
             add-temp-root
             add-indirect-root
             add-permanent-root
@@ -1397,6 +1398,11 @@ When a handler is installed with 'with-build-handler', it is called any time
                                          (message "unsupported build mode")
                                          (status  1))))))))))))
 
+(define-operation (ensure-path (store-path path))
+  "Make PATH a temporary root for the duration of the current session.
+Return #t."
+  boolean)
+
 (define-operation (add-temp-root (store-path path))
   "Make PATH a temporary root for the duration of the current session.
 Return #t."