cmd/tailscale: make tailscale cert --serve-demo accept optional listen argument

Change-Id: I48f2f4f74c9996b9ed4bee02c61f125d42154a34
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f