x/playground: upgrade runsc version

[prattmic]

The root playground fs contains a __runsc_containers__ entry, which was removed from runsc in July 2019: https://play.golang.org/p/_bvQjtzhkuR, google/gvisor@04cbb13

Downloading the runsc binary from https://go.googlesource.com/playground/+/refs/heads/master/sandbox/cloud-init.yaml#15, runsc --version is not supported. This flag was added to runsc in April 2019: google/gvisor@7543e9e

Together, these make it seem like the runsc build is quite old? Once the new playground is stable, we'll want to be sure to update runsc regularly (official releases: https://github.com/google/gvisor/releases) to keep up with security fixes.

[toothrot]

Thanks, @prattmic!

Tagging #25224 for context.

[gopherbot]

Change https://golang.org/cl/295650 mentions this issue: sandbox: always use latest runsc version

[toothrot]

Deployed. We'll now stay up to date when we restart our sandbox instances.