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 [the upcoming `1.2`
release of `poetry`](python-poetry/poetry#3010 (comment)).

But until then let's use the cleaner workaround.

python/helpers/build

@@ -20,7 +20,8 @@ cp -r \
 cd "$install_dir"
 PYENV_VERSION=3.10.5 pyenv exec pip --disable-pip-version-check install --use-pep517 -r "requirements.txt"
 
-# Workaround of https://github.com/python-poetry/poetry/issues/3010
+# TODO remove this workaround of https://github.com/python-poetry/poetry/issues/3010
+# once Poetry 1.2 ships.
 # 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