caching conda environment in github actions

[JuliaKukulies]

Realizing that the installation of our conda requirements in every github action slows down the github actions (see #288), it could be worth using some type of caching of the environment in our workflow.

This is, however, not critical yet and might only become important as we add further actions (e.g., #278).

It is documented here how to do this, but I will have to dive a bit more into this:
https://github.com/conda-incubator/setup-miniconda#caching-environments