health: clean up updateBuiltinWarnablesLocked a bit, fix DERP warnings

Updates #13265

Change-Id: Iabe4a062204a7859d869f6acfb9274437b4ea1ea
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f