The setup.sh script appears to use some non-posix compliant syntax.
Specifically $(("new_number")) in line 80 resulted in an error when using dash as /bin/sh. This commit therefore updates the shebang to use /bin/bash instead.
This is done to make sure the sddm theme folder gets the correct
permissions for sddm to be able to read it.
Users with custom `umask` commands in their .bashrc may suffer
permission denials otherwise.
