Braden was the sitemap.xml file created?
If it worked that way, it should work in the CRON.
If it works on the command line but not in the CRON, check your Flarum log file for errors. If that still doesn't help, you will probably need to try to redirect the output to a log file with > (and also redirect errors with &2> or something like that, I can never remember how to write it) so you can inspect the output.
EDIT: also check which linux user your CRON job runs at. Ideally it should be the same user that you use for SSH login. If it's a different user, it might be unable to write the xml file to disk, and if so will likely not be able to write to the log file either.
