Use DRUSH_BOOTSTRAP_NONE in favour of DRUSH_BOOTSTRAP_DRUSH

[ghost]

There are two mentions of DRUSH_BOOTSTRAP_DRUSH in this repo, yet the main Drush repo specifies DRUSH_BOOTSTRAP_NONE as the bootstrap phase to use for non-boostrapped commands. Indeed, 5 other commands in this repo use DRUSH_BOOTSTRAP_NONE. So we should change the two instances of DRUSH_BOOTSTRAP_DRUSH to DRUSH_BOOTSTRAP_NONE instead.