Cluster_Arch overridable

```suggestion
CLUSTER_OS ?= $(shell $(OC_CLI) get node -o jsonpath='{.items[0].status.nodeInfo.operatingSystem}' 2> /dev/null)
CLUSTER_ARCH ?= $(shell $(OC_CLI) get node -o jsonpath='{.items[0].status.nodeInfo.architecture}' 2> /dev/null)
```
For followup.

_Originally posted by @kaovilai in https://github.com/openshift/openshift-velero-plugin/pull/268#discussion_r1705712797_
            