CI: Add additional beam energies to the CI workflow

It's easy to change the beam energy in the config (`.github/workflows/ci.yml`); we could extend this to test additional beam energies simultaneously, but this would greatly increase the CI load. An idea from the recent SIDIS WG meeting is to test the two extreme beam energies: 5x41 and 18x275, which would double the load. I'm not sure how much is "too much" for the runners, but maybe we can think of a way of testing the different beam energies more efficiently...

This is a good first issue for someone who wants to learn CI configuration, but it may require significant refactoring of `ci.yml`.