allow use of not-embedded pip

[dimbleby]

Fixes #6060

The code still falls back to embedded pip if no other pip can be found - see the implementation of Env.pip().

(Tests change because we save ourselves a call to get_embedded_wheel() which causes a python script to be run to get environment info.)

I realise that the discussion in #6060 hasn't yet reached a conclusion but this is trivial enough that we might as well have it available.

[dimbleby]

seeing #6205 reminded me of this one.

Certainly this doesn't bring such large gains as are claimed there but I reckon it does take about 50% off a typical "poetry install" (with warm caches, downloaded artifacts).

[neersighted]

I'm fine with this, but I'd like to hear from a few others.

[radoering]

I thought, it might make a difference when updating pip as a locked dependency (at least on Windows). But it seems it makes no difference if the embedded pip or the pip inside the venv is used. I always get the following error:

ERROR: To modify pip, please run the following command: ...\python.exe -m pip install ...

So I'm fine with it, too.

[brandon-leapyear]

✨ This is an old work account. Please reference @brandonchinn178 for all future communication ✨

---

Note: it seems like this PR fixed a bug in 1.2.0 where an upgrade to virtualenv would fail poetry self update when trying to call the embedded pip (virtualenv just released a new version today, which is causing the issue).

Repro:

docker run --rm -it python:3.10 bash

# in docker
python3 -m venv venv
venv/bin/pip install poetry==1.2.0 virtualenv==20.16.5
venv/bin/poetry self update

Fails with:

  Command ['/venv/bin/python3', '/venv/lib/python3.10/site-packages/virtualenv/seed/wheels/embed/pip-22.2.2-py3-none-any.whl/pip', 'install', '--disable-pip-version-check', '--prefix', '/venv', '--upgrade', '--no-deps', '/root/.cache/pypoetry/artifacts/8a/85/69/120758446f5ee47cfa1c7c237156c8eaf982af2f9f5778dd1aa1395273/poetry-1.2.2-py3-none-any.whl'] errored with the following return code 2, and output: 
  /venv/bin/python3: can't open file '/venv/lib/python3.10/site-packages/virtualenv/seed/wheels/embed/pip-22.2.2-py3-none-any.whl/pip': [Errno 2] No such file or directory

Opening an issue doesn't seem appropriate since this is now fixed, but maybe a test should be added to ensure this doesn't regress?

[github-actions]

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