Create a working payu environment

[aidanheerdegen]

The conda environment will be deployed with conda-pack, which requires a conda environment file.

So this issue requires creating and testing an environment file.

Note that we wish to use mamba for speed and efficiency reasons, so follow the guidelines for creating a mamba env

https://mamba.readthedocs.io/en/latest/user_guide/mamba.html

It should be pretty straightforward as payu is deployed from the repo

https://github.com/payu-org/payu/blob/master/.github/workflows/CD.yml

to the accessnri channel

https://anaconda.org/accessnri