Cache `conda` for GitHub Actions, and for `act`.

Whenever we set up `conda` in our workflows, we get download it fresh, which takes runtime and bandwidth on our end and on Anaconda's end. We don't necessarily need the latest version.

Figure out how to cache a version of `conda` for:

1. Use within GitHub Actions.
2. Use when running `act` locally. (mount container to local `conda` install)

See https://github.com/crickets-and-comb/shared/issues/45.

Inspect the repository's GitHub Actions workflow files to find every conda setup, then read the linked shared issue 45 for related context. Determine how the same cached conda installation can serve GitHub Actions and an act container; done means workflows avoid downloading it repeatedly and act can use the local installation.

Written by the indexing model from the issue text.