Running Cron with an updated Bash version
This should be easy to sort.
From your shell, examine the full path of the bash you prefer
which bash

Then edit your script in tmp to call the same path to bash (or your script has to source in the dot files, but it’s custom to just make each item in cron know the full proper path and not depend on startup and login files as that is much more light weight for cron)
I’ll end with one last link on good customs for cron environment being intentionally different than an interactive shell.

Use cron to screencapture on Mountain Lion