use hardwired /tmp -- fifo only work on local file-system;

--- a/lib/Tools/mkfifo	Fri Aug 29 08:14:59 2008 +0200
+++ b/lib/Tools/mkfifo	Fri Aug 29 20:36:05 2008 +0200
@@ -29,6 +29,6 @@
 
 [ "$#" != 0 ] && usage
 
-FIFO="${ISABELLE_TMP_PREFIX}-fifo$$"
+FIFO="/tmp/isabelle-fifo$$"
 mkfifo -m 600 "$FIFO" || fail "Failed to create named pipe $FIFO"
 echo "$FIFO"