sandbox: always use latest runsc version

Configure cloud-init.yaml to always use the latest stable runsc release.
This will ensure we get security and bug fixes as we restart our sandbox
instances.

Fixes golang/go#38317

Change-Id: Ib1d763a3256cb2ce37388c700fdbf35fec8d8d80
Reviewed-on: https://go-review.googlesource.com/c/playground/+/295650
Trust: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/sandbox/cloud-init.yaml b/sandbox/cloud-init.yaml
index 41195a4..66e4090 100644
--- a/sandbox/cloud-init.yaml
+++ b/sandbox/cloud-init.yaml
@@ -12,6 +12,6 @@
     }
 
 runcmd:
-- curl -L -o /var/lib/docker/runsc https://storage.googleapis.com/go-builder-data/runsc-1.0.1
+- curl -L -o /var/lib/docker/runsc https://storage.googleapis.com/gvisor/releases/release/latest/x86_64/runsc
 - chmod +x /var/lib/docker/runsc
 - systemctl reload docker.service