fix(python-plugin): add UV_PROJECT_ENVIRONMENT env variable

[apgrucza]

Summary

Fixes #2658 (i.e. errors such as the below):

warning: VIRTUAL_ENV=src/uvtest/.venv does not match the project environment path .venv and will be ignored; use --active to target the active environment instead

I have also updated the outdated docs which still contained the old default virtual environment path of .devbox/virtenv/python.

How was it tested?

Using the steps from #2658:

devbox create uvtest --template python-pip
cd uvtest
devbox add uv@latest
mkdir -p src/uvtest
echo -e "[project]\nname = \"uvtest\"\nversion = \"1.0.0\"\nrequires-python = \">=3.12\"" > pyproject.toml
devbox shell --env VENV_DIR="./src/uvtest/.venv"
uv run main.py

With this fix, the last command now runs successfully.

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