profile: Use _IO* but disable deprecation warning.

This fixes a regression introduced in
2f60084f77, whereby the profile derivation
would fail to run on Guile 2.0 (as is the case with "guix package
--bootstrap").

Reported by Christopher Baines.

* guix/profiles.scm (profile-derivation)[builder]: Use _IO* but add
'debug-disable' call.

guix/profiles.scm

@@ -1268,8 +1268,11 @@ (define builder
                          (guix search-paths)
                          (srfi srfi-1))
 
-            (setvbuf (current-output-port) 'line)
-            (setvbuf (current-error-port) 'line)
+            ;; Don't complain about _IO* on Guile 2.2.
+            (debug-disable 'warn-deprecated)
+
+            (setvbuf (current-output-port) _IOLBF)
+            (setvbuf (current-error-port) _IOLBF)
 
             #+(if locales? set-utf8-locale #t)