health, wgengine/magicsock: remove last of health package globals

Fixes #11874
Updates #4136

Change-Id: Ib70e6831d4c19c32509fe3d7eee4aa0e9f233564
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