The current approach of building the environment does not install the cudatoolkit package. Hence, openmm can not use CUDA.
We can install the openmm8.1 compatible cudatoolkit by 'conda install -c conda-forge cudatoolkit=11.8'. The easiest update would be to add cudatoolkit=11.8 in the yml file itself as it will also be downloaded from the conda-forge channel.