Fix cluster label in backup metrics

[eshutov]

Summary

This PR fixes an issue with the Prometheus metrics sent from the restic backup pod to the pushgateway, specifically concerning the "cluster" label.

The "cluster" label was introduced in #1030. However, this label doesn't currently works as intended. The clusterName values from the backup/schedule manifest are not being passed to the backup pod's environment. As a result, the "cluster" label consistently appears empty ("") in pushgateway/prometheus.

Additionally, there is another point to discuss.
These two PRs #1030 and #260 added ClusterName and PromURL to "restic check" (./api/v1/check_types.go)
code, which are intended for the code that pushes metrics to Pushgateway. It appears the actual code implementing this push functionality is missing, At least, I was unable to find it. So the "check" job doesn't push any metrics.

A reasonable solution might be to completely remove these configurations and clean the CRDs accordingly. But since these would be breaking changes, I have left the existing code unchanged, assuming that the metrics-pushing code either exists elsewhere or will be implemented in the future.

Checklist

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• I have not made any changes in the charts/ directory.