Separate CI build and simulation run

[axel-h]

Separate CI build and simulation run, so the building queue is no blocked by the simulation getting stuck. Currently, if a build is for a platform that has a simulation, the python script in the ci-actions repo will start the simulation immediately after the build. See e.g. https://github.com/seL4/ci-actions/blob/0161435968ab9d2811e478c4c6e32bb0d4f5f4fb/camkes-vm/build.py#L46-L50 how it it implemented at the moment. Once we have split build and simulation, we could also run simulation in parallel then.