ipn/ipnlocal: don't warn about serve listener failing on IPv6-less machines

Fixes #6303

Change-Id: Ie1ce12938f68dfa0533246bbe3b9d7f3e749a243
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