health: fix data race in new warnable code

Fixes #12479

Change-Id: Ice84d5eb12d835eeddf6fc8cc337ea6b4dddcf6c
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