setup-poetry: Hide Poetry installation from tools that search the workspace for Python files

[bkeryan]

What does this Pull Request accomplish?

Move the Poetry installation from $GITHUB_WORKSPACE/.cache/poetry to $GITHUB_WORKSPACE/../.cache/poetry. This hides the Poetry installation from any tools (such as ni-python-styleguide, mypy, pyright, etc.) that search $GITHUB_WORKSPACE for Python files.

The GitHub runner software sets $GITHUB_WORKSPACE to end in my-repo-name/my-repo-name, which is mentioned in the documentation. The outer my-repo-name directory is called the "pipeline directory" in the runner source code and appears to be deleted along with the workspace directory, but isn't really discussed in the documentation.

Why should this Pull Request be merged?

Fixes #33

What testing has been done?

• Ran updated tests before fixing the bug, and ensured that they fail for the right reason, e.g. https://github.com/ni/python-actions/actions/runs/18048801869/job/51365949653

Installing the current project: test-project (0.1.0)
Run poetry run ni-python-styleguide lint
/home/runner/.cache/pypoetry/virtualenvs/test-project-Yn0-klAp-py3.13/lib/python3.13/site-packages/flake8_import_order/styles.py:3: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
  from pkg_resources import iter_entry_points
./.cache/poetry/home/lib/python3.13/site-packages/anyio/__init__.py:1:1: D104 Missing docstring in public package
./.cache/poetry/home/lib/python3.13/site-packages/anyio/__init__.py:27:1: I100 Import statements are in the wrong order. 'from ._core._fileio import open_file' should be before 'from ._core._fileio import Path'
./.cache/poetry/home/lib/python3.13/site-packages/anyio/__init__.py:33:1: I100 Import statements are in the wrong order. 'from ._core._sockets import as_connectable' should be before 'from ._core._sockets import UNIXConnectable'
./.cache/poetry/home/lib/python3.13/site-packages/anyio/__init__.py:68:1: I100 Import statements are in the wrong order. 'from ._core._tasks import CancelScope' should be before 'from ._core._tasks import TASK_STATUS_IGNORED'
./.cache/poetry/home/lib/python3.13/site-packages/anyio/__init__.py:77:1: I100 Import statements are in the wrong order. 'from ._core._tempfile import gettempdir' should be before 'from ._core._tempfile import TemporaryFile'

• Ran updated tests with the fix.
• Tested the fix with nidaqmx-python: https://github.com/ni/nidaqmx-python/actions/runs/18049750895/job/51368950129?pr=845

[bkeryan]

FYI @szb640 I don't have a non-ephemeral self-hosted runner to test with. Could you test this with your workflows?