Demonstrate more of the CI options

[zmitchell]

Our flox/activate-action GitHub Action has options for specifying which environment to activate, but the tutorial doesn't mention those options. This could give the impression that you are only able to activate an environment stored at the top level of your repo, but that's not the case. See https://github.com/flox/activate-action/blob/main/action.yml for more of the available options.