`winutil.WTSGetActiveConsoleSessionId` only works for physical desktop
logins and does not return the session ID for RDP logins. We need to
`windows.WTSEnumerateSessions` and find the active session.
Fixes https://github.com/tailscale/corp/issues/15772
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>