When updating on Windows, we make a copy of the tailscale.exe file in a
temp directory to perform the update, because the original tailscale.exe
gets deleted during the update.
This can eat up disk space if a machine is stuck doing repeated failed
update attempts. Clean up old copies explicitly before making a new one,
same as we do with MSIs.
Updates #10082
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
59d1077e28