logtail, net/portmapper, wgengine/magicsock: use fmt.Appendf

Fixes #5206

Change-Id: I490bb92e774ce7c044040537e2cd864fcf1dbe5a
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