env: update Plan 9 image

This new Plan 9 image includes a fix to the
ramfs file server so that the number of open
file descriptors is correctly initialized
when creating a file.

Fixes golang/go#29033.

Change-Id: I24c536e2d8fc5bf5b2239ecdc01a5ac8fcbe9bdb
Reviewed-on: https://go-review.googlesource.com/c/152037
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
diff --git a/env/plan9-386/make.bash b/env/plan9-386/make.bash
index 8bc88e2..4cf52e7 100755
--- a/env/plan9-386/make.bash
+++ b/env/plan9-386/make.bash
@@ -6,7 +6,7 @@
 
 # Download Plan 9
 if ! sha1sum -c plan9-gce.iso.sha1; then
-  curl --fail -O http://9legacy.org/download/go/2016-10-17/plan9-gce.iso.bz2
+  curl --fail -O http://9legacy.org/download/go/2018-11-30/plan9-gce.iso.bz2
   bunzip2 plan9-gce.iso.bz2
   sha1sum -c plan9-gce.iso.sha1
 fi
diff --git a/env/plan9-386/plan9-gce.iso.sha1 b/env/plan9-386/plan9-gce.iso.sha1
index 43bd35a..ccd1dc2 100644
--- a/env/plan9-386/plan9-gce.iso.sha1
+++ b/env/plan9-386/plan9-gce.iso.sha1
@@ -1 +1 @@
-1afff3c97e67fb2e2db4ebe40597b48e30306fa4  plan9-gce.iso
+b6f362a7483806036cdea26400b64519a6e8f60a  plan9-gce.iso