ssh/tailssh: add envknob for default PATH

As backup plan, just in case the earlier fix's logic wasn't correct
and we want to experiment in the field or have users have a quicker
fix.

Updates #5285

Change-Id: I7447466374d11f8f609de6dfbc4d9a944770826d
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