Build individual environments in Action

[kwinkunks]

Currently we have the test-builds action https://github.com/agile-geoscience/kosu/actions/workflows/test-all.yml

It uses the kosu test command to build the "uber" environment file, which contains all of the packages we need in a single environment. The idea is that if there's a problem, we should find out.

But it would be better to build each of the course-specific environments. We could do this with individual Actions, but I am pretty sure it should be possible with a single action and a matrix strategy (like we use for platforms). I just couldn't see how to do it... the catch is that we need the path for each course in .kosu.yaml.