Fix pythreejs spack build

This sets the npm cache folder to a writable location for the build process

mentioned in merge request !665

@rominabaila thanks! yes, this is probably the easiest way to make the build pass. I was looking for a more permanent fix, because the problem seems to be the way Spack sets the npm_config_cache var in general. For example, if you look at the py-tvb-widgets build env, there is this:

export npm_config_cache=/mnt/spack_v0.23.1/ebrains-spack-builds/vendor/spack/opt/spack/__spack_path_placeholder__/__spack_path_placeholder__/__spack_pat/linux-ubuntu24.04-x86_64/gcc-13.3.0/py-bokeh-2.4.3-ygbe4wodclcrq7ehcqecxpifoatsce4h/npm-cache;

which is obviously not correct.

I am running some tests to figure out if this is just some mis-defined dependencies and dependency types. If they don't succeed in the next couple of hours I'll just merge this and open a more long-term issue for the actual problem :)