Improved support for system environment

[abn]

utils/env: better support system site packages dir
This change improves handling of site-packages under system env, by
gracefully handling fallbacks to user site when required and possible.

Resolves: #3079

This was verified using the following Dockerfile.

FROM python:3.8

RUN useradd -m user
RUN pip install git+https://github.com/abn/poetry.git@issue/3079

USER user
WORKDIR /home/user

RUN poetry config virtualenvs.create false
RUN poetry new foobar

WORKDIR /home/user/foobar

RUN poetry install
RUN poetry run pip show foobar
RUN ls -all /home/user/.local/lib/python3.8/site-packages

editable: use writable script dir for system env

This change ensures that, When using system environment, poetry falls
back to userbase if default location is not writable.

utils/env: ensure we create user directories

[sdispater]

Looks good to me 👍

[github-actions]

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