Without bundle exec, seems to work on a production site 
root@Canapin-app:/var/www/discourse# rake hashtags:mark_old_format_for_rebake
Finding posts matching old format, this could take some time...
[!] You are about to mark 0 posts containing hashtags in the old format to rebake. [CTRL+c] to cancel, [ENTER] to continue
Done, rebakes will happen when periodical updates job runs.