portlist: wait for lsof cmd to exit

We were leaking processes otherwise.

Co-authored-by: Mihai Parparita <mihai@tailscale.com>
Signed-off-by: Maisem Ali <maisem@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