ci: Work around poetry/virtualenv wheel issue

Virtualenv change breaks older Poetry versions. For more background:
python-poetry/poetry#10378
pypa/virtualenv#2883

Using fix mentioned here:
python-poetry/poetry#10378 (comment)

Author: kadler

.github/workflows/build.yml

@@ -34,6 +34,7 @@ jobs:
     - name: Install dependencies
       run: |
         poetry config virtualenvs.create false
+        poetry self add virtualenv@20.30.0 # https://github.com/python-poetry/poetry/issues/10378#issuecomment-2858311161
         poetry install
     - name: Run tests
       run: python -m pytest tests