ssh/tailssh: replace incubator process with su instead of running su as child

This allows the SSH_AUTH_SOCK environment variable to work inside of
su and agent forwarding to succeed.

Fixes #12467

Signed-off-by: Percy Wegmann <percy@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