utils: Make sure to use the right 'bytevector->string'.

In Guile 2.2, (rnrs io ports) exports 'bytevector->string', which
conflicts with that of (ice-9 iconv).

* guix/utils.scm: Hide 'bytevector->string' from (rnrs io ports).

guix/utils.scm

@@ -31,7 +31,7 @@
   #:use-module (srfi srfi-60)
   #:use-module (rnrs bytevectors)
   #:use-module (ice-9 binary-ports)
-  #:autoload   (rnrs io ports) (make-custom-binary-input-port)
+  #:use-module ((rnrs io ports) #:hide (bytevector->string))
   #:use-module ((rnrs bytevectors) #:select (bytevector-u8-set!))
   #:use-module (guix memoization)
   #:use-module ((guix build utils) #:select (dump-port))