Add config setting for regression in Poetry 1.4.1

This is an attempt to solve recently failing builds. Solution borrowed
from:

microsoft/debugpy#1246

Update: fix syntax

Author: delfanbaum

.github/workflows/workflow.yaml

@@ -25,6 +25,8 @@ jobs:
           virtualenvs-in-project: true
           installer-parallel: true
 
+      - run: poetry config installer.modern-installation false
+
       - name: Load cached venv if possible
         id: cached-poetry-dependencies
         uses: actions/cache@v2