explicitly install poetry-plugin-export

[dustinblack]

Changes introduced with this PR

Responding to warning from poetry:

Warning: poetry-plugin-export will not be installed by default in a future version of Poetry.
In order to avoid a breaking change and make your automation forward-compatible, please install poetry-plugin-export explicitly. See https://python-poetry.org/docs/plugins/#using-plugins for details on how to install a plugin.

---

By contributing to this repository, I agree to the contribution guidelines.

[dbutenhof]

Yeah, sure; but I always thought it was weird to "manage dependencies" using poetry but then go out of the way to "install dependencies" using pip anyway. Why not just use poetry install instead of worrying about exporting? (That's what we did for cpt-dashboard instead of adjusting to the new export plugin.)

[webbnh]

Echoing Dave's sentiment, apparently you can also declare the plug-in as a dependency, which might produce an easier-to-maintain result.

If you felt so inclined, you could add a default value (e.g., latest) to the ARG declaration, which would quiet the warning.