fix: activate venv in publish_pypi.sh

[joeyzhao2018]

Summary

• Fix poetry: command not found error in the publish-pypi-package CI job
• The before_script runs setup_python_env.sh which installs poetry inside a virtualenv, but since it executes as a subshell the venv activation doesn't persist into the script: phase
• Add source venv/bin/activate before calling poetry, matching the pattern used by the lint and unit-test jobs