x/build: run cmd/scaleway cron every few minutes to repair world

[bradfitz]

We should have a k8s cron that runs cmd/scaleway in its repair mode every few minutes to check on Scaleway and restart down or wedged instances.

/cc @adams-sarah @cybrcodr

[jessfraz]

sgtm

[gopherbot]

Change https://golang.org/cl/52270 mentions this issue: cmd/scaleway: add kubernetes cronjob to run in repair mode

[gopherbot]

Change https://golang.org/cl/170441 mentions this issue: cmd/scaleway: run instance checker on Kubernetes regularly