import: hackage: Remove reference to unbound variable.

* guix/scripts/import/hackage.scm (guix-import-hackage): Fix error message for
  imports from stdin, which referred to unbound variable 'package-name'.

guix/scripts/import/hackage.scm

@@ -125,8 +125,8 @@ (define (run-importer package-name opts error-fn)
           (()
            (run-importer "stdin" opts
                          (lambda ()
-                           (leave (_ "failed to import cabal file from '~a'~%"))
-                           package-name)))
+                           (leave (_ "failed to import cabal file \
+from standard input~%")))))
           ((many ...)
            (leave (_ "too many arguments~%"))))
         (match args