cmd/derper: fix data race & server panic in manual cert mode

(Thanks for debugging, Roland!)

Fixes #4082

Change-Id: I400a64001c3c58899bb570b759b08e745abc0be1
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