Apptainer integration tests for HPC

[nmerket]

Is your feature request related to a problem? Please describe.

We have some basic tests in test_eagle.py for testing the mechanics of running on HPC, but they're somewhat limited and don't catch much. We have to mock the simulations themselves in the containers and any calls to the job scheduler (slurm).

Describe the solution you'd like

Install Apptainer (formerlly known as Singularity) on CI to actually do the building simulations in our tests. We would still have to mock the slurm stuff, but that would be significantly less and allow us to test out how simulations will run in our HPC environments.

Describe alternatives you've considered

Just keep what we have.

Additional context

Sort of related to or inspired by #385.