x/build: use cloud build to update docker images

[toothrot]

A number of our buildlets on Google Cloud Platform run in docker images. Rather than running manual commands for each image, we could kick off periodic cloud build jobs to build images, and then update builder configurations as necessary.

Regularly updating images would also help us match what user's are more likely using, and also allow us to create builders that are up to date with upcoming OS releases.

[gopherbot]

Change https://go.dev/cl/449037 mentions this issue: env/linux-x86-sid: add cloudbuild configuration

[gopherbot]

Change https://go.dev/cl/460355 mentions this issue: cmd/coordinator: deploy the coordinator with Cloud Build