EVM Node: Fix the `octez_evm_node_head` metrics for the RPC node

Prior the this changeset, the metrics is only set in the apply_blueprint routine, something the RPC mode never calls. We propose to add it to the main event loop of the RPC mode as well. We don’t fix its companion octez_evm_node_confirmed_head for now, as it’s less interesting for the RPC mode (it does not influence the behavior of the RPC mode).

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
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• Provide automatic testing (see the testing guide).
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