ipn/ipnlocal: generate tailscaled-owned SSH keys as needed

Updates #3802

Change-Id: Ie1bc9ae3f3639603b88b4e19b7eb12bea528ff77
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