Use mamba instead of conda in openmc install script

I am switching the openmc_setup.sh script to use mamba to install instead of conda for installation. An error with installing OpenMC via conda has been an on and off issue for a while. It did not seem to affect this install process previously, but I tried setting up a new Workbench instance the other day and the error had returned. The solution around this installation error is to use mamba instead to install which is more robust for handling these incompatibility errors.