Check for dependency changes in CI

### What problem does your feature request solve?

In our PR template we have the line
> Conda lock files have been updated if dependencies have changed.

This is a bit meaningless to most changes.

### Describe the solution you'd like

We could use an additional GitHub Actions check to only comment if the lock files have changed. This would probably just involve hashing the conda yaml files, and maybe using the cache action as a database. We might also need to refresh this hash in the conda update action.

### Describe alternatives you've considered

The current manual check does work, but we should be getting computers to do repetitive work.

Start by inspecting the PR template and the existing conda update GitHub Actions workflows. Determine how conda YAML and lock files are currently handled; done means CI comments only when dependencies changed, with the conda update workflow keeping any comparison state current.

Written by the indexing model from the issue text.