env: update Plan 9 image

This new image provides a new tool, called
aux/randfs. This is a pseudo-random file
system which can be mounted on top of /dev/random.

We noticed the Go binaries on GCE were running
much slower than usual. It happened because the
runtime was regularly waiting after /dev/random,
which wasn't able to produce random bytes as
fast as required.

A simple workaround was to write a pseudo-random
file system, acting as /dev/random and initialized
from a true random source. It will be able
to produce much more bytes per second than
the random generator provided by the kernel.

Update golang/go#9491

Change-Id: I81158adef7332d10295c2245b4564ae18ebb3a14
Reviewed-on: https://go-review.googlesource.com/6170
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

env/plan9-386/make.bash

@@ -6,7 +6,7 @@ set -e
 
 # Download Plan 9
 if ! sha1sum -c plan9-gce.iso.sha1; then
-  curl --fail -O http://9legacy.org/download/go/2015-01-25/plan9-gce.iso.bz2
+  curl --fail -O http://9legacy.org/download/go/2015-02-26/plan9-gce.iso.bz2
   bunzip2 plan9-gce.iso.bz2
   sha1sum -c plan9-gce.iso.sha1
 fi
@@ -180,6 +180,12 @@ send "echo echo remove 10.0.0.0 /104 10.0.0.0 '>'/net/iproute >>/cfg/helix/cpurc
 expect -exact "term% "
 send "echo ramfs -u >>/cfg/helix/cpustart\n"
 
+expect -exact "term% "
+send "echo aux/randfs -m /dev >>/cfg/helix/cpustart\n"
+
+expect -exact "term% "
+send "echo >>/cfg/helix/cpustart\n"
+
 expect -exact "term% "
 send "echo echo downloading git >>/cfg/helix/cpustart\n"
 

env/plan9-386/plan9-gce.iso.sha1

@@ -1 +1 @@
-bd6130449b742ecc5f558153676ee660f66806f6  plan9-gce.iso
+d277abfb4431ae1ca2cbf93d3c0b2cacdad6f166  plan9-gce.iso