net/portmapper: add envknob to disable portmapper in localhost integration tests

Updates #11962

Change-Id: I8212cd814985b455d96986de0d4c45f119516cb3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f