Disable Poetry experimental new installer

python-poetry/poetry#3811
python-poetry/poetry#4210

Poetry installs are erroring with `JSONDecodeError`s in GitHub Actions:

`poetry cache clear --all pypi --no-interaction` does not fix the issue.
Removing the actions/cache step for Poetry fixes the issue.
Disabling the experimental new installer also fixes the issue.

Disable installer with `poetry config experimental.new-installer false`,
which updates poetry.toml.

Additionally, the setting is not correctly implemented as a Boolean, so
`export POETRY_EXPERIMENTAL_NEW_INSTALLER=false` does not disable the
installer (python-poetry/poetry#3811). It can be set to an empty string,
or disabled using `poetry config experimental.new-installer false` and
updating poetry.toml.

This commit will disable the experimental new installer in poetry.toml.

poetry.toml

@@ -1,2 +1,5 @@
+[experimental]
+new-installer = false
+
 [virtualenvs]
 in-project = true