Workaround thread safety issue by limiting worker count

The original intent of this code was to workaround a thread safety
issue. However, according to the `poetry` maintainer, the more "future
proof" workaround is [to limit the worker count rather than disable the
new
installer](python-poetry/poetry#3010 (comment)).

The underlying root issue is supposedly resolved in [newer versions of
`poetry`](python-poetry/poetry#3010 (comment)),
which I will do in a follow-on PR. But wanted to first update the
workaround so that if we need to revert the upgrade we still have the
better workaround in place.

python/helpers/build

@@ -23,4 +23,4 @@ PYENV_VERSION=3.10.5 pyenv exec pip install -r "requirements.txt"
 # Workaround of https://github.com/python-poetry/poetry/issues/3010
 # By default poetry config file is stored under ~/.config/pypoetry
 # and is not bound to any specific Python version
-PYENV_VERSION=3.10.5 pyenv exec poetry config experimental.new-installer false
+PYENV_VERSION=3.10.5 pyenv exec poetry config installer.max-workers 1