ssh/tailssh: flesh out env, support non-pty commands

Updates #3802

Change-Id: I7022460117542a5424919144828bf571c7c19ec0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f