Use Cluster and not Manager for build clusters

[alvaroaleman]

At the time controller-runtime was introduced to this project, it didn't have a clean abstraction for "cache + client of a cluster". The only thing it had was the manager, which is that plus some additional things like metrics, webhooks, leader election and code related to run things. As a result, we ended up using the manager and disabling everything it doesn't need.

By now, controller-runtime has the cluster abstraction for precisely this use-case. This change makes us use the cluster where appropriate. It should not result in any outwards-visible behavior changing.

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[matthyx]

/lgtm /hold

[droslean]

/lgtm /approve /hold cancel

Thanks @alvaroaleman !!!!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alvaroaleman, droslean

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/prow/blob/main/OWNERS)~~ [alvaroaleman,droslean] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment