control/controlclient: do not alias peer CapMap

Updates #cleanup

Change-Id: I10fd5e04310cdd7894a3caa3045b86eb0a06b6a0
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f