Add missing env update in SConstruct when PREFIX is not set

[clemisch]

Fixes #25

[sbillinge]

@clemisch @vincefn do we know why this suddenly showed up, when it wasn't failing before? I'm happy to merge it if we don't think it breaks any other installation paths, but it would be nice to understand why it suddenly popped up after all these years.

[ezatterin]

Perhaps no one tried to build the library outside of a conda env or anything else exporting a PREFIX env variable :) Indeed if one is in a python venv running export CONDA_PREFIX=$VIRTUAL_ENV and building with sudo -E to preserve the env variables works fine.

[sbillinge]

yes, makes sense. We are heading to a new release to catch the python 3.14 update, so I am happy to merge this. We will have to test that it doesn't break anything for the conda builds.