When building the project, specifically when building SpiderMonkey bundle, the python script doesn't correctly isolates the virtualenvironment and read dependencies installed outside it (for instance the User site packages). Having the wrong version of the dependencies on those other locations can end up in SpiderMonkey crashing at build.
The documentation for the environtment variable PYTHONNOUSERSITE can be found here: https://docs.python.org/3/using/cmdline.html#envvar-PYTHONNOUSERSITE. Basically, setting the env var to a non-empty value makes Python to avoid using packages installed within the user-site packages, having to rely on the packages installed within the system-site packages, or the ones installed within the virtual environment.
Setting things this way, avoids conflicts with python dependencies installed by the users in their home directories, and makes the installation script to download the correct versions of it's dependencies.