Previously we only set this when it updated, which was fine for the first
call to Start(), but after that point future updates would be skipped if
nothing had changed. If Start() was called again, it would wipe the peer API
endpoints and they wouldn't get added back again, breaking exit nodes (and
anything else requiring peer API to be advertised).
Updates tailscale/corp#27173
Signed-off-by: James Sanderson <jsanderson@tailscale.com>