CI: Use srun instead of sbatch

As a workaround, we currently use sbatch instead of srun as mpirun via cmake --target test via srun does not work out of the box. There seem to be some flags / environment variables which could make srun work.

Idea:

# Have to unset SLURM_* variables (except SLURM_CONF) to get mpi working
srun -c 4 --mem-per-cpu=4G --account=es_kochm --time=01:00:00 \
  bash -c '
    conf="$SLURM_CONF" ; unset ${!SLURM_*} ; export SLURM_CONF="$conf" ;
    cmake --build build --target test
  '