env: add -s flag to ramfs in Plan 9 image

We add -s flag to ramfs to post its channel
on /srv/ramfs, so multiple clients could
access its files.

Updates golang/go#28485.

Change-Id: Id39b17fcc3bec2b5fae3a16841ab2414e60361b3
Reviewed-on: https://go-review.googlesource.com/c/146038
Reviewed-by: Bryan C. Mills <[email protected]>

Author: 0intro

env/plan9-386/make.bash

@@ -184,7 +184,10 @@ expect -exact "term% "
 send "echo echo remove 10.0.0.0 /104 10.0.0.0 '>'/net/iproute >>/cfg/helix/cpurc\n"
 
 expect -exact "term% "
-send "echo ramfs -u >>/cfg/helix/cpustart\n"
+send "echo ramfs -su >>/cfg/helix/cpustart\n"
+
+expect -exact "term% "
+send "echo mount -c /srv/ramfs /tmp >>/cfg/helix/cpustart\n"
 
 expect -exact "term% "
 send "echo aux/randfs -m /dev >>/cfg/helix/cpustart\n"