substitute: Warn upon store prefix mismatches.

Suggested by Hynek Urban <hynek.urban@gmail.com>.

* guix/scripts/substitute.scm (fetch-narinfos): Move body to...
[do-fetch]: ... here.  New procedure.
Emit a warning when CACHE-INFO's prefix does not match.

guix/scripts/substitute.scm

@@ -565,13 +565,7 @@ if file doesn't exist, and the narinfo otherwise."
              (read-to-eof port))
          result))))
 
-  (define cache-info
-    (download-cache-info url))
-
-  (and cache-info
-       (string=? (cache-info-store-directory cache-info)
-                 (%store-prefix))
-       (let ((uri (string->uri url)))
+  (define (do-fetch uri)
     (case (and=> uri uri-scheme)
       ((http)
        (let ((requests (map (cut narinfo-request url <>) paths)))
@@ -589,7 +583,19 @@ if file doesn't exist, and the narinfo otherwise."
          (filter-map (cut narinfo-from-file <> url) files)))
       (else
        (leave (_ "~s: unsupported server URI scheme~%")
-                   (if uri (uri-scheme uri) url)))))))
+              (if uri (uri-scheme uri) url)))))
+
+  (define cache-info
+    (download-cache-info url))
+
+  (and cache-info
+       (if (string=? (cache-info-store-directory cache-info)
+                     (%store-prefix))
+           (do-fetch (string->uri url))
+           (begin
+             (warning (_ "'~a' uses different store '~a'; ignoring it~%")
+                      url (cache-info-store-directory cache-info))
+             #f))))
 
 (define (lookup-narinfos cache paths)
   "Return the narinfos for PATHS, invoking the server at CACHE when no