Needed for an upcoming complex neuron demo on BSS-2.

Suggested by CINECA.

Previously, we used a custom do_fetch() to download some additional files
needed for the build and to prepare the build folder.
Now, we use spack to track all resources (i.e. download the additional files
itself) and only setup the build folder in install().

This reverts commit 674e26c0

This reverts commit 017e5e23