derp: fix fmt verb for nodekeys

It was hex-ifying the String() form of key.NodePublic, which was already hex.
I noticed in some logs:

    "client 6e6f64656b65793a353537353..."

And thought that 6x6x6x6x looked strange. It's "nodekey:" in hex.

Updates tailscale/corp#20844

Change-Id: Ib9f2d63b37e324420b86efaa680668a9b807e465
Signed-off-by: Brad Fitzpatrick <bradfitz@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