profiles: Adjust for compatibility with Guile 2.0.5.

Reported by Andreas Enge <andreas@enge.fr>.

* guix/profiles.scm (right-arrow): Use 'set-port-conversion-strategy!'
  instead of '%default-port-conversion-strategy'.  The latter is only
  available in Guile 2.0.5.

guix/profiles.scm

@@ -321,10 +321,10 @@ replacement if PORT is not Unicode-capable."
     (let ((arrow "→"))
       (catch 'encoding-error
         (lambda ()
-          (with-fluids ((%default-port-conversion-strategy 'error))
-            (with-output-to-string
-              (lambda ()
-                (display arrow)))))
+          (call-with-output-string
+            (lambda (port)
+              (set-port-conversion-strategy! port 'error)
+              (display arrow port))))
         (lambda (key . args)
           "->")))))