Install tests don't seem to use the environment provided by
`def setup_run_environment(…)`; we work around by providing the
necessary PYTHONPATH environment variable manually.

Until now we used a lot of try/except to keep going when preparing the
environment for the build; it should just work now, so let's fail early.

This should fix problems with finding system-installed headers instead of the
spack-provided ones (e.g. googletest). (In some cases sites might inject
include paths via compiler settings (or other means)).

We allow for newer py-pybind11 versions than upstream; upstream pins a specific
version, but, e.g., arbor needs a newer one. Currently, this should just work
fine :).

We encountered build fails on some HPC sites due to openssl mismatches
between bare-metal tools and spack — especially curl and similar tools
for fetching.
Our build tool, waf, needs to find spack-provided libraries, e.g.
libclang.so, at runtime… so we will only set the environment variable
for this.

Co-Authored by: Eleni Mathioulaki <emathioulaki@athenarc.gr>

Do not keep py-pyaescrypt and py-formencode at the top level listed