Use ESMF Action in CI as alternative to Conda

[angus-g]

This action has just been released https://github.com/esmf-org/install-esmf-action, which we could use as an alternative to using Conda in the CI environment. Probably not a high priority and wouldn't make much of a difference, but it could give a little bit more control over the actual ESMF backend we're using.