substitute: Gracefully handle trailing slashes in URLs.

Previously, using something like
"--substitute-urls=http://example.org///" would lead to a
'cache-narinfo!' call with #f as its second argument.

It would also do the wrong thing for URLs with a non-empty initial path
component, such as "http://example.org/foo/bar".

* guix/scripts/substitute.scm (fetch-narinfos)[handle-narinfo-response]:
Add call to 'basename' for PATH.

guix/scripts/substitute.scm

@@ -610,7 +610,8 @@ if file doesn't exist, and the narinfo otherwise."
             (update-progress!)
             (cons narinfo result))
           (let* ((path      (uri-path (request-uri request)))
-                 (hash-part (string-drop-right path 8))) ; drop ".narinfo"
+                 (hash-part (basename
+                             (string-drop-right path 8)))) ;drop ".narinfo"
             (if len
                 (get-bytevector-n port len)
                 (read-to-eof port))