TST: Need to replace a GitHub action

[nkeim]

CI tests are failing because of a change in GitHub actions:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v2, s-weigand/setup-conda@v1. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

Changing to checkout@v4 is easy enough, but the conda action has not been updated. We need to switch to another one, which may entail changing how we describe the environments.