Make sure uv installs into the right virtualenv

[davisagli]

If a different virtualenv is active while running make install, then uv will detect it and install there instead of into .venv. (See https://docs.astral.sh/uv/pip/environments/#using-arbitrary-python-environments) Then other commands fail, such as in plone/cookieplone#38

To avoid this we can explicitly set the VIRTUAL_ENV environment variable. (See https://docs.astral.sh/uv/pip/environments/#using-arbitrary-python-environments)

I tested and this avoids the problem that happened in plone/cookieplone#38

[yurj]

ok for me

[loechel]

LGTM, Thanks for the fix

[petschki]

I can report, that this works with a python -m venv venv virtualenv. However my special case with pyenv virtualenv plugin still doesn't work (same error again), but I would say that's an edge case and blame it on this special environment likely not following the python virtualization standards. So generally approved from me ...

[petschki]

UPDATE: forget my comment about pyenv virtualenv ... I forgot to pick this changes before make install it in this environment 🙈

It also works for pyenv virtualenv environments like it should!

So thanks for this one!