ipn/ipnserver: provide means of setting debug env vars on Windows

Fixes #3688

Change-Id: I2f88ca32bf764e2b084fe544d821c43b6a20ffa5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f