utils: wrap-program: Refuse to wrap .X-real files.

* guix/build/utils.scm (wrap-program): Error if wrap-program was
mistakenly passed a .X-real file. This prevents and forces us to fix
cases where a double wrapped ..X-real-real file is created, such as can
be seen with: "find /gnu/ -iname '.*-real-real'".

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

guix/build/utils.scm

@@ -1303,6 +1303,9 @@ with definitions for VARS."
        (format #f "export ~a=\"$~a${~a:+:}~a\""
                var var var (string-join rest ":")))))
 
+  (when (wrapped-program? prog)
+    (error (string-append prog " is a wrapper. Refusing to wrap.")))
+
   (if already-wrapped?
 
       ;; PROG is already a wrapper: add the new "export VAR=VALUE" lines just