config: fix handling of venv opts env vars

[abn]

This change correctly handles virtual environment creation options provided via environment variables. For example, previously when POETRY_VIRTUALENVS_OPTIONS_SYSTEM_SITE_PACKAGES was specified via an environment variable, this was incorrectly ignored when a virtual environment was created.

Reproduction Instruction

podman run --rm -i --entrypoint bash docker.io/python:3.12 <<EOF
set -xe
python -m pip install --disable-pip-version-check -q git+https://github.com/python-poetry/poetry.git
export POETRY_VIRTUALENVS_OPTIONS_SYSTEM_SITE_PACKAGES=true
export POETRY_VIRTUALENVS_IN_PROJECT=true
poetry new demo
pushd demo
poetry install
cat .venv/pyvenv.cfg
EOF

Current Behaviour

The key include-system-site-packages is set to false.

home = /usr/local/bin
implementation = CPython
version_info = 3.12.2.final.0
virtualenv = 20.25.1
include-system-site-packages = false
base-prefix = /usr/local
base-exec-prefix = /usr/local
base-executable = /usr/local/bin/python3.12
prompt = demo-py3.12

After Fix Behaviour

podman run --rm -i --entrypoint bash docker.io/python:3.12 <<EOF
set -xe
python -m pip install --disable-pip-version-check -q git+https://github.com/python-poetry/poetry.git@refs/pull/9015/head
export POETRY_VIRTUALENVS_OPTIONS_SYSTEM_SITE_PACKAGES=true
export POETRY_VIRTUALENVS_IN_PROJECT=true
poetry new demo
pushd demo
poetry install
cat .venv/pyvenv.cfg
EOF

The key include-system-site-packages is correctly set to true.

home = /usr/local/bin
implementation = CPython
version_info = 3.12.2.final.0
virtualenv = 20.25.1
include-system-site-packages = true
base-prefix = /usr/local
base-exec-prefix = /usr/local
base-executable = /usr/local/bin/python3.12
prompt = demo-py3.12

[github-actions]

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