pin arbor to latest version

With Spack 0.18.1, the concretizer prefers to use py-pybind11@2.8.1 (probably because the latest versions of py-scipy are not compatible with py-pybind11@2.10.1) and for this reason installs arbor@0.5.2. Pinning the arbor version to 0.8 in spack.yaml solves that.