[v18] Fixes possible collisions when hashing immutable labels

[eriktate]

Backports #63733 to branch/v18

Manual testing

• Provision a scoped token assigning immutable labels: tctl scoped tokens add --type=node --scope=/local --assign-scope=/local --ssh-labels=aaa=bbbcccddd
• Confirm nodes are still able to join and heartbeat with immutable labels
• Restart the node and confirm it retains its immutable labels and heartbeats are successful
• Stop the node and modify the immutable labels in process storage to be:

{
  "immutable_labels": {
    "ssh": {
      "aaa": "bbb",
      "ccc": "ddd"
    }
  }
}

• Restart the node and confirm it fails to register its inventory control stream with an auth error complaining about immutable labels not matching the cert's label hash