blender 2.82: python environment still not picked up by add-ons

[amurrayc]

Describe the bug
As noted in #81218, since the upgrade to blender 2.82 the PYTHONPATH environment isn't checked any more:

blender patch

This means that, for example, numpy is not detected and so import numpy at the python console (in blender) produces a "not found" error. Also, add-ons that use numpy fail to load.

In this blender issue two workarounds are suggested.
#81313 proposes a solution using the second of these. However, while this allows import numpy at the console it appears that add-on loading happens before the --python-expr command is executed and so it still fails if the add-on depends on numpy.

I have a pull-request using the first solution proposed above which seems to work for me.

To Reproduce
Steps to reproduce the behavior:

1. Install Blender normally.
2. Download the zipfile from BlenderGIS and install it (under Edit/Preferences/Add-ons/Install...)
3. Enable the add-on. (This should work)
4. Quit and restart Blender.
5. Observe the numpy-not-found error (although Blender will start)
6. (Optionally) Note that in the Scripting tab import numpy works and that toggling the add-on brings it to life.

Expected behavior
Blender should start happily with the add-on enabled.

Additional context
I had the problem with the Archipack Pro addon but BlenderGIS is free so it's better for testing.

Metadata

• system: "x86_64-linux"
• host os: Linux 5.5.8, NixOS, 20.03beta482.b780aefc621 (Markhor)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.2
• channels(root): "nixos-20.03beta482.b780aefc621, nixos-unstable-20.09pre215333.42f0be81ae0"
• channels(murray): "home-manager"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
- blender
# a list of nixos modules affected by the problem
module: