cmd/tailscaled: respect $PORT on all platforms, not just Linux

Updates #5114

Change-Id: I6c6e28c493d6a026a03088157d08f9fd182ef373
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