Use canonical service account for ccb service account

[tylerphelan]

We use a ccb-service-account in the build-service namespace for ccb service account: https://github.com/pivotal/build-service-cli/blob/master/pkg/commands/customclusterbuilder/create.go#L71

This couples the CLI to the build-service namespace. This service account can probably be the canonical.service-account and can be in the kpack namespace.