install poetry-plugin-export manually in tests workflow [WIP]

[ArneBinder]

to resolve poetry deprecation warning from ci (during Run Nox step):

nox > poetry export --format=requirements.txt --with=dev --without-hashes
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.
To disable this warning run 'poetry config warnings.export false'.

Warning: This does not seem to work as expected, the warning is still shown. Needs some further investigation.

EDIT: This should use this instead when available in new poetry release: python-poetry/poetry#9547