Instead of overloading the Version field, add an explicit Track field.
This fixes a bug where passing a track name in `args.Version` would keep
the track name in `updater.Version` and pass it down the code path to
commands like `apt-get install`. Now, `updater.Version` should always be
a version (or empty string).
Updates #cleanup
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>